Locally built sing-box v1.13.21 carries no version stamp and reports
'unknown', leaving the panel core-version column empty. The agent probe
now falls back to /etc/nexusgate/sing-box/version when the binary
reports no version; agent-singbox.sh writes that record (version
without the v prefix) on every successful install path, including the
already-installed early exit. Agent version stays 0.6.8.
agent-go: engineReport gains singBoxVersion. The probe runs
`sing-box version` once, parses the version token from its first line
('unknown' from unstamped builds reports empty), and caches it keyed by
the binary's mtime+size — re-probed only when an engine install/update
replaces the file. Steady-state heartbeats pay one stat, no subprocess.
Agent version stays 0.6.8.
panel-go: EngineState.singBoxVersion persisted from heartbeats (old
agents without the field keep showing '-'), servers table core-version
column now renders it. Golden gains heartbeatWithEngine +
serversAfterEngineHeartbeat sections locking the full chain, plus a
listen-address (<hostip>) normalization for the sandbox bind address.
singboxversion_test.go covers parsing, cache hits, and re-probe on
binary replacement.
aaaamoooo