feat(wire): split the frame discriminant into trait and method bytes

[decrypto21]

Draft: not for merge yet. Tracking: #351

What

• The frame discriminant becomes two u8s instead of one: the API trait (namespace) and the method within that trait.
  • before: [SCALE str requestId][u8 id][payload]
  • after: [SCALE str requestId][u8 trait][u8 method][payload]
• Trait ids are explicit literals via a new trait-level #[wire_trait(id = N)] attribute, carried through the macro, rustdoc extraction, and both emitters. Missing or duplicate trait ids are hard codegen errors; there is no fallback that derives an id from declaration or module order.
• Method ids restart from 0 within each trait, preserving each trait's existing relative order. The existing derivation rules are unchanged, now scoped per trait: a request takes n/n+1, a subscription takes n..n+3. Append-only becomes a per-trait rule.
• Codec version goes 1 to 2 (--codec-version default, scripts/codegen.sh, the generated client constant, and the handshake's acceptance check).
• Unknown discriminant pairs and undecodable frames now fail visibly instead of being dropped silently.

Why

• The flat u8 is the whole address today, so new methods can only append at the global tail. Five traits already have non-contiguous id blocks as a result (account spans 18-28, 110-112, and 164), and ids 70-75 are permanently burned by the retired JsonRpc trait. 163 of 255 values are already assigned, and a single subscription costs four slots.
• Splitting gives every trait its own 256-slot method space, keeps each trait's block contiguous permanently, and puts the namespace on the wire where dispatch and debug tooling can see it.
• Since this is a coordinated cutover, existing ids are renumbered once and append-only resumes per trait. Preserving the old global numbers would carry today's fragmentation into the new scheme for no compatibility benefit: the envelope grows a byte, so no existing peer can parse a new frame regardless of which numbers it carries.

Compatibility

• A peer on codec 1 talking to a new host gets an explicit UnsupportedProtocolVersion rather than a hang.
• The reverse direction cannot be rescued by the version bump: the handshake frame itself rides the changed envelope, so an old peer cannot parse a new host's reply well enough to learn it is incompatible. It fails on its own side.
• The codec bump plus the loud-failure changes exist so mismatches surface as errors instead of the silent-drop-then-hang failure this protocol produced the last time a byte layout moved under an unchanged version tag.
• Phone pairing and the inter-host SSO messages are untouched: that layer carries a string message_id with its own codec indices and has no reference to this envelope's discriminant.

Verification

• cargo test --workspace --all-features: 648 passed, 0 failed. cargo fmt --all -- --check, cargo clippy --workspace --all-targets --all-features -- -D warnings, and the wasm32-unknown-unknown build of truapi-server are all clean.
• js/packages/truapi: tsc build clean, bun test 206 passed, 0 failed. js/packages/truapi-host: 40 passed.
• Playground: yarn build and yarn lint clean against a refreshed package snapshot.
• Byte-level goldens were recomputed rather than relaxed, and a new test pins the handshake frame end to end so trait 0 stays the system trait. Both per-byte range guards and both truncation errors (missing trait byte, missing method byte) have distinct-message tests in Rust and TypeScript.
• The Rust and TypeScript wire tables are compared against each other by an existing parity test, now including trait ids.
• Reviewed across multiple adversarial lenses: the old-to-new mapping was independently re-derived for all 160 wire actions and matches the documented appendix row for row, and every generated artifact was confirmed byte-reproducible from the branch sources.