perf: expose FBUILD_WATCH_SET_CACHE_SECS env override for the watch-set cache window #122

Follow-up from #120.

DaemonWatchSetCache uses a hardcoded 2 s freshness window (DEFAULT_FRESHNESS in crates/fbuild-daemon/src/watch_set_cache.rs). For CI / benchmark scenarios where builds happen in rapid succession OR development sessions where the user edits slowly, the value should be tunable without a rebuild — mirroring the FBUILD_SELF_EVICTION_SECS override added in #116.

Scope:

• Read FBUILD_WATCH_SET_CACHE_SECS at daemon startup.
• Pass the parsed Duration to DaemonWatchSetCache::with_max_age(...) in DaemonContext::with_hub.
• Document the env var in crates/fbuild-daemon/README.md alongside the other FBUILD_* toggles.
• Optional: 0 means "disable the cache" — bypass the DashMap entirely so regressions are easy to A/B.

Related: #120 (the cache itself), #116 (same pattern for self-eviction).