The agent only reported chrony as synced|unsynced — nothing about how well the
clock tracks. Parse `chronyc tracking` for the source-relative metrics.
- proto: HealthSnapshot.chrony_offset_seconds (16, signed system-time offset,
fast=+), chrony_source (17, reference name/IP), chrony_stratum (18); regenerated.
- agent: replace chronyState() with chronyTracking()/parseChrony() — one exec,
parses Leap status + Reference ID + Stratum + System time offset (+ test).
- manager: Health.{ChronyOffset,ChronySource,ChronyStratum}; snapshotToHealth maps
them; influx writes chrony_offset_seconds/_stratum/_source and whitelists the
offset + stratum as graphable fields.
- UI: chronySummary() + offsetMs() — the chrony row now shows
"synced · 192.168.50.10 · s3 · +0.01ms" instead of just "synced".