v0.5.0-rc1: slice 5 Phase A — storage observe/report + watchdog (read-only, live)

Fill the slice-3 storage_targets stub and add the fast-poll storage watchdog.
Read-only this phase; the host-root surface (mounts/SMART/grow/destructive gate)
is Phase B. Hub-owned desired manifest is slice 10, so reconcile against it is
built-but-unfed.

- internal/storage: StorageTarget wire contract, durable_id derivation per type,
  HostReader seam (procfs/sysfs, root-free), Observer (storage_targets from
  ListStorage/NodeStorage + host reads, lvmthin thin-pool fill), and the watchdog
  (third daemon goroutine; debounced out-of-band report on a known target's
  attach/disconnect transition).
- proxmox.Storage: additive parse-only config fields (durable_id sources).
- collector StorageObserver seam; Loop.SetTrigger out-of-band report; daemon runs
  the watchdog as a third goroutine; StorageConfig knobs.
- cross-repo golden kept byte-identical with felhom.eu/hub; bidirectional key-set test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/hub/loop_test.go

@@ -122,6 +122,33 @@ func TestLoop_RunImmediateAndResilientAfterError(t *testing.T) {
 	}
 }
 
+func TestLoop_OutOfBandTriggerReportsImmediately(t *testing.T) {
+	// The storage watchdog's trigger channel runs an extra report between ticks (the slow
+	// cadence is 1h here, so any report within the window comes from the trigger).
+	var cn, rn int32
+	loop := NewLoop(
+		&fakeCollector{report: &HostReport{}, n: &cn},
+		&fakeReporter{env: &ControlEnvelope{}, n: &rn},
+		time.Hour, quietLogger())
+	trigger := make(chan struct{}, 1)
+	loop.SetTrigger(trigger)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan error, 1)
+	go func() { done <- loop.Run(ctx) }()
+
+	// Immediate report fires first (1 collect). Then fire the trigger → one more report.
+	time.Sleep(20 * time.Millisecond)
+	trigger <- struct{}{}
+	time.Sleep(20 * time.Millisecond)
+	cancel()
+	<-done
+
+	if got := atomic.LoadInt32(&cn); got < 2 {
+		t.Errorf("collect calls = %d, want ≥2 (immediate + out-of-band trigger)", got)
+	}
+}
+
 func TestLoop_RunAdoptsSlowerInterval(t *testing.T) {
 	var cn, rn int32
 	loop := NewLoop(