RING-01: the A2A protocol in T27, from the contract that exists

[gHashTag]

Part of #1279. Ring 1 — the A2A protocol. The contract already exists and is not to be redesigned: nine routes in agent-server/apps/server/src/api/routes/a2a.ts, the store in pg-agent-store.ts, the client in rings/SR-02/A2ARegistryClient.swift.

The ring states the types and the semantics: agent card, message, task, state transitions, and what "alive" means in terms of heartbeat.

Done when

• t27c gen-rust rings/T27-01/a2a.t27 produces Rust that compiles.
• The types match the agents table field for field (id, name, capabilities, last_heartbeat, status, metadata) and match A2AMessage in Swift.
• Task states are exactly those in AgentTaskState.
• The ring contains no I/O: Postgres and HTTP are ring 3, not this one.

Boundary

rings/T27-01/a2a.t27

[gHashTag]

Done, written directly rather than by a bee: the swarm cannot dispatch at all while the provider key is unreachable, and this ring does not depend on that.

rings/T27-01/a2a.t27 generates 152 lines of Rust that compile under bare rustc --crate-type lib. Integer constants rather than enums, for the reason ring 00 gives — the seed's enum lowering is not generation-safe until t27#2247 lands, and the local t27c does not carry that fix yet.

Against the acceptance criteria:

• Generates compiling Rust — yes, and make t27-rings now runs its rules as a table rather than only compiling it.
• Types match the store — online/offline status and the 30-second heartbeat come from pg-agent-store.ts; message kinds mirror A2AMessageType.
• Task states are exactly AgentTaskState — pending, assigned, inProgress, completed, failed, cancelled, in that order.
• No I/O — Postgres and HTTP stay in ring 3.

Three rules are worth reading, because each is a defect that has happened somewhere in this project:

Cancelling is always allowed for unfinished work — the operator's escape hatch, and the one transition that must never be refused. Three missed beats before an agent is offline, not one: marking it dead for a single hiccup makes the registry flap, and a status field that flaps is one every consumer learns to ignore. An out-of-range priority is refused rather than clamped, because a task that says 99 and is read as critical jumps every queue on a typo.

A hole my own break-test found in my own test. Removing the terminal guard from the ring left every row green, because the destinations I looped over did not include TASK_CANCELLED — exactly the one the "always allowed" rule would have let through for finished work. A terminal state must refuse every destination, including the unconditional one. With that row added the break goes red immediately.