feat(daemon): status.daemon control request and bg status health snapshot

Adds DaemonHealthSnapshot (src/daemon/health.rs), built from atomics and
try_lock'ed maps: per-family pending sequencer entries by kind, the
oldest entry's age, the front entry's kind and whether an older open
trace root fences it, open/finishing/written mutating trace roots,
in-flight side-effect passes, side-effect errors, causal-fence release
counters, the existing ingest-loss counters (now also the source of the
stats.ingest response), and a sequencer_stalled flag for work older
than the fence's hard cap. A contended map is reported as
snapshot_partial rather than waited on, and fence state is observed
through the same classification the drain uses without applying any
release, so a status probe never changes what the drain will do.

The snapshot is served by the new status.daemon control request and
printed by git-ai bg status under an additive "daemon" key (with a
restart hint under "daemon_error" when an older daemon does not know
the request), so a fenced or stalled family can be inspected locally and
in tests without waiting for the heartbeat.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>