feat(wire): #31 · Hello capability discovery + the five evolution rules

[hellno]

What

Implements #31 — wire-contract evolution: Hello capability discovery + the five evolution rules. One additive request on the frozen deckard-contract wire, plus the in-repo rules + registry that govern how the contract may grow. This is the keystone the x402 chain (#32 → #34) and the ProposalOrigin::Dapp variant (#198 / #204) register their new kinds against.

SignerRequest::Hello  ->  SignerResponse::Hello(HelloInfo {
    spec_version: "YYYY-MM-DD",     // date-version; bumps only on a breaking change
    capabilities: Vec<String>,      // e.g. ["core", "mcp.v0.1"] — feature detection reads THIS only
    impl_name:    String,           // informational; no code path branches on it
})

An old daemon that predates the variant fails to decode it and returns the existing frame-decode error — that rejection is the backward-compat valve, and it already ships. No existing frame changes.

How it maps to #31's in-scope checklist

• ✅ Hello + HelloInfo in deckard-contract; answered in every daemon state incl. Locked (the daemon arm reads no self.state); reveals capability names only — no state, policy, or key material.
• ✅ Baseline capability registry (core, mcp.v0.1) — in code (crates/deckard-contract/src/capabilities.rs, the single source of truth) and as the name · status · since · defining doc table in the new build doc.
• ✅ The five evolution rules written into docs/build/40-wire-evolution.md (indexed in docs/build/README.md).
• ✅ Tests:
  • E1 Hello shape — spec_version matches ^\d{4}-\d{2}-\d{2}$, capabilities ⊇ {core, mcp.v0.1}.
  • E2 old-peer replay — existing frames encode byte-identically (golden-pinned unit frames + the Balance data frame; the round-trip fixtures were extended with Hello, every prior assertion unchanged).
  • E3 unknown enum variant → decode Err, no panic (CBOR + JSON), and at the socket the daemon replies malformed_request, keeps serving, and signs nothing.
  • E4 extra unknown struct key → decodes, key ignored (no deny_unknown_fields).
• ✅ No code path branches on impl_name.

Single-source parity

The daemon (IMPL_SIGNERD), the MCP acceptance mock (IMPL_MOCK), and the contract MockSigner::hello() all build their answer from one capabilities::hello_info(), so spec_version + capabilities can never drift between implementations — only impl_name differs. Pinned by hello_info_is_single_sourced, hello_matches_the_single_source_registry, and the daemon-socket test.

Open question — deferred (not auto-closing #31)

#31 asks whether HelloInfo should also carry the daemon's active chain id (leaning yes). I did not include it: the normative in-scope text says "capability names only — no state," chain id is state, and a wrong-chain client is already refused at first Propose with Deny{chain_mismatch}. It can be added additively later — precisely by these rules (a new field an old client ignores per E4) — if we decide first-contact chain detection is worth the state it exposes. Flagged in 40-wire-evolution.md; easy to flip if you'd prefer it in now.

Definition of done (evidence)

• cargo fmt --all --check — clean.
• just check — green (clippy -D warnings on default, --features tray, and --features dev-signerd-bin).
• TMPDIR=/tmp cargo test --workspace — green (430 passed, 2 pre-existing ignored).
• No new or changed dependencies (Cargo.toml / Cargo.lock untouched).
• Adversarial review: three independent lenses over the final diff (codex GPT-5 xhigh + two Claude lenses — wire-compat correctness and security/trust-core). Zero P0/P1. The P2/P3 findings (a false "MockSigner participates" doc claim, a stale test-name reference, and a tautological Balance self-comparison in E2) are all fixed in this branch.

Contract + daemon + client/test work only; no GPUI app views touched.