feat(server): add open session core

Add `irtt-server` with the deterministic protocol and session engine the
upcoming Tokio runtime will call. `ServerCore::handle_datagram` takes a source
endpoint and the bytes of one datagram and returns the bytes to send back, or
nothing. It performs no I/O and keeps no clock, so every decision is a pure
function of its state and that input apart from drawing session tokens.

Rejection is silence. The protocol defines no error reply, no reset and no
NACK, so malformed, unauthenticated and policy-refused datagrams are discarded
without a response and without disturbing any live session; a client tells
"rejected" from "lost" only by timing out. `ServerError` is reserved for
internal failures on the server's own side, so an inbound `ProtoError` never
surfaces as a server failure.

Open handling authenticates before decoding parameters, so a packet that fails
authentication is discarded without parsing untrusted data and no reply
behavior reveals which stage rejected it. Duration and Interval are accepted
when absent and rejected when explicitly present and not positive, which is
what the presence-aware parameter decode is for. Negotiation currently rewrites
only the protocol version; the restriction knobs the specification describes
are deliberately left for their own slices, with one obvious place to add them.

Sessions are bounded to a finite default. A single unauthenticated datagram
creates one and opens are never deduplicated, so an unbounded table is a
liability rather than a compatibility feature; upstream's absence of session
bounds is explicitly not a target. A full table drops new opens silently and
evicts nothing. No-test opens keep working at capacity because they create no
state.

Session tokens come from the operating system's random source, since a token is
the only thing standing between an off-path attacker and a usable session on an
unauthenticated server. A drawn value that is zero or collides with a live
session is redrawn within a finite budget; there is no predictable fallback,
and a colliding token never overwrites the session that holds it. The reply is
encoded before the session is inserted, so an internal failure cannot leave a
half-created session behind.

Echo and close requests pass admission and are then deliberately ignored: this
slice creates sessions, and lifecycle and receive semantics belong to the
slices that implement them. Tests pin that so a half-implemented lifecycle
cannot appear by accident.

crates/irtt-server/AGENTS.md

@@ -0,0 +1,77 @@
+# `irtt-server` guidance
+
+Repository-wide guidance in the root `AGENTS.md` also applies here.
+
+## Where server behavior comes from
+
+Server wire behavior comes from the sanitized clean server documents, not from
+upstream implementation material:
+
+- `docs/protocol/IRTT_SERVER_PROTOCOL_SPEC.md`
+- `docs/protocol/test-vectors/SERVER_BEHAVIORAL_VECTORS.md`
+- `docs/protocol/BLACKBOX_VERIFICATION_REPORT.md`
+- `docs/protocol/IRTT_CLIENT_PROTOCOL_SPEC.md` for the shared wire format
+
+Never derive behavior from upstream `heistp/irtt` source or tests. If the clean
+material does not establish a semantic, state the ambiguity instead of guessing.
+
+## Boundaries
+
+- Protocol behavior comes from `irtt-proto`. `irtt-server` must **not** depend
+  on `irtt-client` to reuse wire logic; a needed primitive belongs in
+  `irtt-proto`, where both sides get it.
+- Do not reimplement magic validation, flag rules, HMAC placement, request
+  classification, token/header offsets or parameter parsing here.
+- No statistics, CLI or presentation concerns.
+- `SocketAddr` belongs here: endpoint identity is session state. It does not
+  belong in `irtt-proto`.
+
+## Core versus runtime
+
+- Deterministic protocol and session logic — admission, negotiation, the session
+  table, resource decisions, packet construction — belongs in the core.
+- Socket and runtime orchestration — `UdpSocket`, IPv4/IPv6 listeners,
+  `recv_from`/`send_to`, timers, expiry, shutdown, `select!`, socket options —
+  belongs around the core.
+- The server is intentionally **Tokio-native**. Tokio does not need to be
+  optional for this crate.
+- Do not add a blocking server, an alternate-runtime variant, a transport trait,
+  a runtime abstraction layer, or a mirrored client-style API family. There is
+  no product requirement for any of them, and `ServerCore` being public is not a
+  promise of runtime independence.
+
+## Rejection is silence
+
+Malformed, unauthenticated, unsupported and policy-refused input is discarded
+without a reply and without disturbing any live session. The protocol defines no
+error reply, no reset and no NACK. Reserve the error type for internal failures
+on the server's own side; an inbound `ProtoError` must never surface as a server
+failure. Hostile UDP must leave the server running.
+
+Do not let reply behavior reveal which stage rejected a datagram — in
+particular, an authentication failure must be indistinguishable from an unknown
+token.
+
+## Resource policy
+
+Total session and resource state must stay bounded. A single unauthenticated
+datagram creates a session and opens are never deduplicated, so an unbounded
+table is a liability. Upstream's observed absence of session and per-peer bounds
+is explicitly **not** a compatibility target and must not be reproduced.
+
+Keep fallible preparation before irreversible state mutation: a reply is
+prepared before a session is inserted, so an internal failure cannot leave a
+half-created session.
+
+## Testing
+
+- Drive the core with real packets built by the production encoders. Do not
+  maintain shadow encoders for well-formed wire data.
+- Hand-build packets only where the point is a payload a compliant encoder
+  cannot produce — truncated varints, out-of-range enums, corrupted MACs.
+- Token generation is the only nondeterministic part; tests inject a scripted
+  source so identity, collisions and allocation failure are assertable. Keep
+  that seam private.
+- As the server gains behavior, prefer it over fake peers for normal compliant
+  behavior in other crates' tests. Keep small raw/adversarial peers for
+  intentionally malformed or non-compliant wire behavior.