Supervise the sync companion independently

[myobie]

Summary

• add independent systemd and launchd supervision for fabric-sync
• keep the embedded engine as the production owner while the companion sends compatibility heartbeats
• report standby, absent, and incompatible companion states in service status, Fabric status, sync status, and doctor
• keep configured sync entries visible when the daemon is down
• restart and uninstall the matched service pair in the required order

Verification

• cargo check --all-targets
• library: 494 passed, 4 ignored
• binary tests: 19 passed
• companion integration: 4 passed
• provisioning integration: 11 passed
• permanent five-second latency window: 502 samples, source max 10.0235 ms, delivery max 18.755291 ms, 8 scans
• both generated launchd plists passed plutil -lint

The embedded sync engine remains active. This change does not cut or deploy a release.