guest-power: add the liveness observable it shipped without (v0.109.0)

The v0.107.0 watchdog was silent on a healthy box, so its health could only be inferred
from absence — F-OBS's shape, shipped in the same session F-OBS was fixed. INFO summary
every 10th sweep with what it saw; aborted sweeps are not counted. Red-proofs 7 and 8.

internal/localapi/server.go

@@ -266,7 +266,11 @@ type Server struct {
 	// guestPower (F-REBOOT) is per-guest start-attempt state for the guest-power watchdog.
 	// Guarded by guestPowerMu in guestpower.go; in-memory on purpose (see guestPowerState).
 	guestPower map[int]guestPowerState
-	host       storage.HostReader // role classification source (optional; defaults to ProcHostReader)
+
+	// guestPowerSweeps counts completed guest-power sweeps, for the liveness observable. Touched only
+	// from GuestPowerTick, which the ticker calls serially.
+	guestPowerSweeps int
+	host             storage.HostReader // role classification source (optional; defaults to ProcHostReader)
 
 	hostMetrics  HostMetricsProvider // slice 9 (optional)
 	hostID       string              // slice 10B: for the data-bearing-format pending-op hint