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.go

@@ -30,6 +30,7 @@ type Loop struct {
 	client    reporter
 	interval  time.Duration
 	logger    *slog.Logger
+	trigger   <-chan struct{} // optional: an out-of-band report request (storage watchdog)
 }
 
 // NewLoop builds the loop. interval is the starting cadence (the hub may override it
@@ -41,6 +42,12 @@ func NewLoop(collector collectorIface, client reporter, interval time.Duration,
 	return &Loop{collector: collector, client: client, interval: interval, logger: logger}
 }
 
+// SetTrigger wires an out-of-band report channel. A receive on it runs one extra
+// collect→report cycle immediately WITHOUT disturbing the regular ticker cadence — used by
+// the storage watchdog to push a disconnect to the hub in seconds. The watchdog debounces,
+// so this fires at most once per debounce window.
+func (l *Loop) SetTrigger(ch <-chan struct{}) { l.trigger = ch }
+
 // Run reports immediately, then on each tick, until ctx is cancelled (then nil).
 func (l *Loop) Run(ctx context.Context) error {
 	interval := l.interval
@@ -60,6 +67,17 @@ func (l *Loop) Run(ctx context.Context) error {
 				interval = next
 				ticker.Reset(interval)
 			}
+		case <-l.trigger:
+			// Out-of-band report (storage watchdog). Run a cycle now; keep the regular
+			// cadence (do not reset the ticker). The envelope's interval is still adopted
+			// if it changed, mirroring the normal path.
+			l.logger.Info("hub: out-of-band report triggered (storage watchdog)")
+			next := l.cycle(ctx, interval)
+			if next != interval {
+				l.logger.Info("hub: poll interval changed", "from", interval, "to", next)
+				interval = next
+				ticker.Reset(interval)
+			}
 		}
 	}
 }