perf: watch-set cache hit/miss telemetry for validating savings in the field #123

Follow-up from #120.

DaemonWatchSetCache currently has zero observability — a user who enables the cache has no way to tell whether it actually short-circuited the fp-watches-collect walk on their project. For tuning the freshness window (#122) and diagnosing stale-hit confusion, we should emit counters.

Scope:

• On get: emit tracing::debug!(target: "fbuild_daemon::watch_set_cache", hit = true|false, key_hash, age_ms, "watch-set cache lookup").
• Aggregate cumulative hit / miss counters on DaemonContext exposed via /api/daemon/info (already the home for similar aggregate stats).
• Add an integration test that runs two back-to-back builds and asserts the second one bumps the hit counter.

Non-goals:

• No Prometheus / OTLP export. Plain tracing + JSON field on /api/daemon/info is enough for now.

Related: #120 (the cache), #91 (warm-build instrumentation).