feat(project): Establish initial IRTT workspace and protocol definitions

Set up the multi-crate project structure and add the core IRTT protocol with pinned dependencies for reproducible builds.

crates/irtt-proto/src/error.rs

@@ -0,0 +1,35 @@
+use thiserror::Error;
+
+pub type Result<T> = std::result::Result<T, ProtoError>;
+
+#[derive(Debug, Error, PartialEq, Eq)]
+pub enum ProtoError {
+    #[error("packet is too short: needed {needed} bytes, got {actual}")]
+    PacketTooShort { needed: usize, actual: usize },
+    #[error("bad magic bytes")]
+    BadMagic,
+    #[error("reserved flag bits are set: 0x{0:02x}")]
+    ReservedFlags(u8),
+    #[error("required flag 0x{0:02x} is missing")]
+    MissingFlag(u8),
+    #[error("unexpected flag 0x{0:02x} is set")]
+    UnexpectedFlag(u8),
+    #[error("HMAC flag/key mismatch")]
+    HmacPresenceMismatch,
+    #[error("HMAC verification failed")]
+    BadHmac,
+    #[error("invalid HMAC field offset")]
+    InvalidHmacOffset,
+    #[error("varint is truncated")]
+    TruncatedVarint,
+    #[error("varint is too long")]
+    VarintOverflow,
+    #[error("string value is not valid UTF-8")]
+    InvalidUtf8,
+    #[error("invalid enum value {value} for {name}")]
+    InvalidEnum { name: &'static str, value: i64 },
+    #[error("open reply has zero token without close flag")]
+    ZeroToken,
+    #[error("trailing or malformed parameter payload")]
+    MalformedParams,
+}