Foundational building blocks for the Honcho sync path rework. Provider
integration lands in a follow-up commit; this commit is standalone unit-
tested primitives that expose clean seams for the integration tests:
SyncWorker
Persistent daemon thread draining a bounded task queue. Replaces
the per-turn threading.Thread(target=_sync).start() pattern so
sync_turn() returns immediately, never coordinating thread handoff
on the user-facing path. Queue overflow drops the OLDEST task
(with on_failure callback) rather than blocking the producer —
preserves responsiveness under load.
HonchoLatencyTracker
Rolling p95 observer with a warmup phase (returns default until
N samples collected) and a floor. timeout() = max(floor, headroom
* p95). Hosted Honcho settles to ~1-3s; self-hosted cold starts
scale up naturally. No hardcoded 30s ceiling visible to users.
CircuitBreaker
Closed → Open after N consecutive failures. Probe interval lets
Open → HalfOpen one request through; success closes, failure
reopens. Thread-safe. Time source is injectable for determinism
in tests.
24 tests covering all three primitives plus their integration (worker
feeds latency observations to the tracker and success/failure to the
breaker; breaker-open enqueue drops the task via on_failure).
No behavioural change to runtime yet — nothing in the codebase
imports these primitives in this commit.
