Adds an atomic degraded-until state (health.go: IsDegraded/MarkDegraded/
MarkHealthy) that lets request handlers fail fast with a clear
unavailable error instead of blocking on xcecli's own ~100s retry
budget when the backend is known-degraded.
Consolidates what tasks 4.3/4.4 left as duplicated inline
"call xcecli_*; checkAndPerformRejoin()" pairs across five wrapper
functions into one shared callBackend() helper: checks IsDegraded()
before every call (fail fast, skip the C call and the rejoin check
entirely on the degraded path -- checking rejoin there would itself
block up to ~100s, defeating the fail-fast purpose), marks degraded on
-E_XCE_NOT_READY, marks healthy at checkAndPerformRejoin()'s existing
success point (the single call site both the write path and the
idle-period heartbeat path already funnel through).
The circuit breaker's own TTL-based state is deliberately never read
by the rejoin-retry mechanism -- checkAndPerformRejoin() has no
IsDegraded() dependency, so a fast-fail window never suppresses
retry-until-actually-joined; heartbeat.go's unconditional per-tick
call keeps that cadence alive regardless of degraded state, and a
10s TTL (well under the ~100s retry budget) lets write traffic resume
contributing once it clears.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NNB4zGPCsHyDUCDJyMbiQN
Rongsong Shen
c8325e7575
