Render readiness: the gate and the air-state hook

[robert-dean]

Split from #29. Pieces 1 and 2 are built: a render that fails because its engine is unavailable is released for retry (releaseForRetry in segment.repository.ts) and the planner retries it (BreakPlanner.retryRenderOf).

Two deferred items remain:

• Piece 3, a readiness gate the render job consults before starting, rather than a reordering of modules.ts. Only named today, in a comment in speech.gate.ts. A render that lost the race with plugin startup is failed as if it were wrong #29 ranked it weakest of the three and last.
• A host-side air-state hook for speech plugins. Chatterbox unloads its model after unloadAfterIdleMinutes of no synthesis, whether or not the station is on air. The plugin SDK has no air-state signal to tell it otherwise.

The arguments for deferring both are in #29.

[robert-dean]

Status, 2026-10-24. The air-state hook is built (#770).

• The hook. host.air on every plugin host: isOn(), and onChange() once per change. "On air" is PlayoutPusher.wantsAir(), a programme and an audience together. The pusher publishes it as air.wanted on the station bus, and PluginAirState in plugins follows it. The audience gate's linger is the debounce, so a listener dropping out for a moment does not move it. Listeners are called outside any invocation, a throw is logged and never counts toward quarantine, and disposing a plugin drops its listeners.
• Chatterbox uses it. The idle unload counts down only while the station is off air. It starts when the station stands down and never drops a model the plugin did not load. On air, the model is kept however long the station goes between breaks. Coming on air also loads the model ahead of the first break, sharing one load with speak.

Still open: piece 3, the readiness gate, deferred on #29's argument.

[robert-dean]

Closing. The air-state hook is built (#770). Piece 3, the readiness gate, is not going to be built: pieces 1 and 2 already make the race with plugin startup survivable (a break is retried rather than written off), and a gate that judged wrong would fail closed on a station with no speech plugin at all. The argument is in #29 § "The shape of the fix", and the gate stays named in a comment in speech.gate.ts if a measured case ever reopens it.