refactor(proto): preserve open parameter presence

`Params::decode` seeded `Params::default()` and overwrote only the tags it
saw, so an omitted tag and one explicitly encoded as zero were
indistinguishable afterwards. The protocol gives those the same value but not
the same meaning, and a server cannot apply its open admission rules without
telling them apart: an absent Duration or Interval is accepted as the wire
default zero, while one explicitly encoded as zero is invalid.

Add `Params::decode_with_presence`, returning the decoded values alongside a
`ParamPresence` recording which known tags appeared. `Params::decode` now
delegates to it, so there is still exactly one parser and the two cannot
diverge. Unknown tags stay ignored and untracked, and repeated known tags keep
last-value-wins.

Add `Clock::Unspecified = 0` as the default, which is the wire default for an
absent Clock tag and what an empty parameter payload decodes to. An explicit
Clock tag encoding zero remains an invalid enum value, so `Unspecified` is
only ever reached by omission; `encode` omits the tag for it rather than
emitting an explicit zero. It selects no clock, so `has_wall`/`has_mono` are
false and no timestamp field is laid out.

`ClientConfig::clock` is public and therefore now constructible as
`Unspecified`, which is a wire default rather than a client mode. Reject it
during config validation. The client default stays `Clock::Both`, so normal
client-generated params and wire vectors are unchanged.

crates/irtt-proto/src/params.rs

@@ -11,6 +11,11 @@ pub const MAX_SERVER_FILL_BYTES: usize = 32;
 /// Higher-level callers should validate user or configuration input before
 /// encoding. The normal `irtt-client` configuration path enforces
 /// [`MAX_SERVER_FILL_BYTES`] for `server_fill`.
+///
+/// [`Params::default`] is the **wire** default: every integer field is zero and
+/// [`clock`](Params::clock) is [`Clock::Unspecified`], which is what an open
+/// request with an empty parameter payload means. It is not a set of sensible
+/// client settings; a client builds its request from its own configuration.
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 pub struct Params {
     pub protocol_version: i64,
@@ -63,21 +68,67 @@ impl Params {
     ///
     /// This includes invalid enum values, malformed UTF-8, and `server_fill`
     /// values longer than [`MAX_SERVER_FILL_BYTES`].
+    ///
+    /// Absent parameters take their wire default, so a caller cannot tell an
+    /// omitted tag from one explicitly encoded as zero. Use
+    /// [`decode_with_presence`](Params::decode_with_presence) when that
+    /// distinction matters; both share one parser.
     pub fn decode(input: &[u8]) -> Result<Self> {
+        Self::decode_with_presence(input).map(|decoded| decoded.params)
+    }
+
+    /// Decodes parameters and additionally reports which known tags appeared.
+    ///
+    /// This is the same parser and the same validation as [`decode`], with the
+    /// presence of each known tag retained. A receiver needs it because the
+    /// protocol gives an omitted tag and an explicit zero the same value but
+    /// not the same meaning: an absent Duration or Interval is accepted as the
+    /// wire default zero, while one explicitly encoded as zero is invalid.
+    ///
+    /// Presence means the tag appeared at least once. Repeated known tags keep
+    /// last-value-wins, and unknown tags remain ignored and untracked.
+    ///
+    /// [`decode`]: Params::decode
+    pub fn decode_with_presence(input: &[u8]) -> Result<DecodedParams> {
         let mut params = Self::default();
+        let mut presence = ParamPresence::default();
         let mut pos = 0;
         while pos < input.len() {
             let (tag, used) = varint::decode_uvarint(&input[pos..])?;
             pos += used;
             match tag {
-                1 => params.protocol_version = read_int(input, &mut pos)?,
-                2 => params.duration_ns = read_int(input, &mut pos)?,
-                3 => params.interval_ns = read_int(input, &mut pos)?,
-                4 => params.length = read_int(input, &mut pos)?,
-                5 => params.received_stats = ReceivedStats::try_from(read_int(input, &mut pos)?)?,
-                6 => params.stamp_at = StampAt::try_from(read_int(input, &mut pos)?)?,
-                7 => params.clock = Clock::try_from(read_int(input, &mut pos)?)?,
-                8 => params.dscp = read_int(input, &mut pos)?,
+                1 => {
+                    params.protocol_version = read_int(input, &mut pos)?;
+                    presence.protocol_version = true;
+                }
+                2 => {
+                    params.duration_ns = read_int(input, &mut pos)?;
+                    presence.duration_ns = true;
+                }
+                3 => {
+                    params.interval_ns = read_int(input, &mut pos)?;
+                    presence.interval_ns = true;
+                }
+                4 => {
+                    params.length = read_int(input, &mut pos)?;
+                    presence.length = true;
+                }
+                5 => {
+                    params.received_stats = ReceivedStats::try_from(read_int(input, &mut pos)?)?;
+                    presence.received_stats = true;
+                }
+                6 => {
+                    params.stamp_at = StampAt::try_from(read_int(input, &mut pos)?)?;
+                    presence.stamp_at = true;
+                }
+                7 => {
+                    params.clock = Clock::try_from(read_int(input, &mut pos)?)?;
+                    presence.clock = true;
+                }
+                8 => {
+                    params.dscp = read_int(input, &mut pos)?;
+                    presence.dscp = true;
+                }
                 9 => {
                     let (len, used) = varint::decode_uvarint(&input[pos..])?;
                     pos += used;
@@ -97,6 +148,7 @@ impl Params {
                         .to_owned();
                     pos += len;
                     params.server_fill = Some(ServerFill { value });
+                    presence.server_fill = true;
                 }
                 _ => {
                     let (_, used) = varint::decode_uvarint(&input[pos..])?;
@@ -104,10 +156,38 @@ impl Params {
                 }
             }
         }
-        Ok(params)
+        Ok(DecodedParams { params, presence })
     }
 }
 
+/// Decoded parameters together with which known tags the payload carried.
+///
+/// Produced by [`Params::decode_with_presence`].
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct DecodedParams {
+    /// The decoded values. Absent parameters hold their wire default.
+    pub params: Params,
+    /// Which known tags appeared in the payload.
+    pub presence: ParamPresence,
+}
+
+/// Which known parameter tags a decoded payload carried.
+///
+/// A field is `true` when its tag appeared at least once, whatever value it
+/// carried. Unknown tags are ignored and are not represented here.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct ParamPresence {
+    pub protocol_version: bool,
+    pub duration_ns: bool,
+    pub interval_ns: bool,
+    pub length: bool,
+    pub received_stats: bool,
+    pub stamp_at: bool,
+    pub clock: bool,
+    pub dscp: bool,
+    pub server_fill: bool,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ServerFill {
     pub value: String,
@@ -179,12 +259,31 @@ impl TryFrom<i64> for StampAt {
     }
 }
 
+/// Which server clock sources supply timestamp fields.
+///
+/// # The zero value
+///
+/// [`Clock::Unspecified`] is the **wire default**, meaning the Clock tag was
+/// absent from an open parameter payload — which is valid, and is what an empty
+/// payload decodes to. It does not mean a peer may send an explicit Clock tag
+/// encoding zero: [`TryFrom<i64>`](Clock::try_from) still rejects an explicit
+/// zero as an invalid enum value, so this state is only ever reached by
+/// omission. [`encode`](Params::encode) omits the tag for `Unspecified` rather
+/// than emitting an explicit zero, which round-trips absence faithfully.
+///
+/// `Unspecified` selects no clock, so [`has_wall`](Clock::has_wall) and
+/// [`has_mono`](Clock::has_mono) are both false for it and no timestamp field
+/// is laid out. A client that wants timestamps must request a real clock; the
+/// `irtt-client` configuration path rejects `Unspecified`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
 #[repr(i64)]
 pub enum Clock {
+    /// Wire default for an absent Clock tag. Never produced from an explicit
+    /// encoded value.
+    #[default]
+    Unspecified = 0,
     Wall = 1,
     Monotonic = 2,
-    #[default]
     Both = 3,
 }
 
@@ -201,6 +300,11 @@ impl Clock {
 impl TryFrom<i64> for Clock {
     type Error = ProtoError;
 
+    /// Converts an **explicitly encoded** Clock value.
+    ///
+    /// Zero is rejected: it is only valid as an absent tag, never as an encoded
+    /// one. [`Clock::Unspecified`] is therefore unreachable through this
+    /// conversion by design.
     fn try_from(value: i64) -> Result<Self> {
         match value {
             1 => Ok(Self::Wall),
@@ -244,6 +348,13 @@ mod tests {
         encoded
     }
 
+    fn encode_int(tag: u64, value: i64) -> Vec<u8> {
+        let mut encoded = Vec::new();
+        varint::encode_uvarint(tag, &mut encoded);
+        varint::encode_varint(value, &mut encoded);
+        encoded
+    }
+
     #[test]
     fn params_round_trip() {
         let params = Params {
@@ -452,6 +563,166 @@ mod tests {
                 value: 0,
             })
         );
+        assert_eq!(
+            Params::decode_with_presence(&encoded),
+            Err(ProtoError::InvalidEnum {
+                name: "Clock",
+                value: 0,
+            })
+        );
+    }
+
+    #[test]
+    fn explicit_clock_values_one_to_three_are_accepted() {
+        for (value, expected) in [(1, Clock::Wall), (2, Clock::Monotonic), (3, Clock::Both)] {
+            let decoded = Params::decode_with_presence(&encode_int(7, value)).unwrap();
+            assert_eq!(decoded.params.clock, expected);
+            assert!(decoded.presence.clock);
+        }
+    }
+
+    #[test]
+    fn empty_payload_decodes_to_wire_defaults_with_nothing_present() {
+        let decoded = Params::decode_with_presence(&[]).unwrap();
+
+        assert_eq!(decoded.params, Params::default());
+        assert_eq!(decoded.params.protocol_version, 0);
+        assert_eq!(decoded.params.duration_ns, 0);
+        assert_eq!(decoded.params.interval_ns, 0);
+        assert_eq!(decoded.params.length, 0);
+        assert_eq!(decoded.params.dscp, 0);
+        assert_eq!(decoded.params.received_stats, ReceivedStats::None);
+        assert_eq!(decoded.params.stamp_at, StampAt::None);
+        // The wire default clock is zero, not `Both`.
+        assert_eq!(decoded.params.clock, Clock::Unspecified);
+        assert!(!decoded.params.clock.has_wall());
+        assert!(!decoded.params.clock.has_mono());
+        assert_eq!(decoded.params.server_fill, None);
+
+        assert_eq!(decoded.presence, ParamPresence::default());
+        assert_eq!(
+            decoded.presence,
+            ParamPresence {
+                protocol_version: false,
+                duration_ns: false,
+                interval_ns: false,
+                length: false,
+                received_stats: false,
+                stamp_at: false,
+                clock: false,
+                dscp: false,
+                server_fill: false,
+            }
+        );
+    }
+
+    #[test]
+    fn absent_and_explicitly_zero_parameters_share_a_value_but_not_a_presence() {
+        // Duration and Interval are the pair a server must tell apart: absent is
+        // accepted, explicitly zero is not.
+        let absent = Params::decode_with_presence(&encode_int(1, 1)).unwrap();
+        assert_eq!(absent.params.duration_ns, 0);
+        assert!(!absent.presence.duration_ns);
+        assert_eq!(absent.params.interval_ns, 0);
+        assert!(!absent.presence.interval_ns);
+        assert_eq!(absent.params.clock, Clock::Unspecified);
+        assert!(!absent.presence.clock);
+
+        let explicit_duration = Params::decode_with_presence(&encode_int(2, 0)).unwrap();
+        assert_eq!(explicit_duration.params.duration_ns, 0);
+        assert!(explicit_duration.presence.duration_ns);
+
+        let explicit_interval = Params::decode_with_presence(&encode_int(3, 0)).unwrap();
+        assert_eq!(explicit_interval.params.interval_ns, 0);
+        assert!(explicit_interval.presence.interval_ns);
+    }
+
+    #[test]
+    fn every_known_tag_reports_its_own_presence() {
+        let mut encoded = Vec::new();
+        for (tag, value) in [
+            (1, 1),
+            (2, 5),
+            (3, 6),
+            (4, 7),
+            (5, 3),
+            (6, 4),
+            (7, 3),
+            (8, 184),
+        ] {
+            encoded.extend_from_slice(&encode_int(tag, value));
+        }
+        encoded.extend_from_slice(&encode_server_fill_value(b"rand"));
+
+        let decoded = Params::decode_with_presence(&encoded).unwrap();
+        assert_eq!(
+            decoded.presence,
+            ParamPresence {
+                protocol_version: true,
+                duration_ns: true,
+                interval_ns: true,
+                length: true,
+                received_stats: true,
+                stamp_at: true,
+                clock: true,
+                dscp: true,
+                server_fill: true,
+            }
+        );
+    }
+
+    #[test]
+    fn unspecified_clock_is_encoded_by_omission() {
+        let params = Params {
+            protocol_version: 1,
+            clock: Clock::Unspecified,
+            ..Params::default()
+        };
+        let encoded = params.encode();
+
+        assert_eq!(encoded, encode_int(1, 1));
+        assert!(
+            !encoded.windows(2).any(|bytes| bytes[0] == 7),
+            "an unspecified clock must omit the tag rather than encode an explicit zero"
+        );
+        assert_round_trip(params);
+    }
+
+    #[test]
+    fn decode_and_decode_with_presence_agree() {
+        let mut payloads = vec![
+            Vec::new(),
+            encode_int(1, 1),
+            encode_int(2, 0),
+            encode_server_fill_value(b"rand"),
+        ];
+        payloads.push(
+            Params {
+                protocol_version: 1,
+                duration_ns: 3_000_000_000,
+                interval_ns: 1_000_000_000,
+                length: 1472,
+                received_stats: ReceivedStats::Both,
+                stamp_at: StampAt::Both,
+                clock: Clock::Both,
+                dscp: 184,
+                server_fill: Some(ServerFill {
+                    value: "rand".to_owned(),
+                }),
+            }
+            .encode(),
+        );
+        // Malformed payloads must fail identically through both entry points.
+        payloads.push(vec![1, 0x80]);
+        payloads.push(encode_int(7, 0));
+
+        for payload in payloads {
+            assert_eq!(
+                Params::decode(&payload),
+                Params::decode_with_presence(&payload).map(|decoded| decoded.params),
+                "decode and decode_with_presence disagreed on {payload:02x?}"
+            );
+        }
     }
 
     #[test]