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/testdata/host-report.golden.json

@@ -29,7 +29,63 @@
       "controller_version": ""
     }
   ],
-  "storage_targets": [],
+  "storage_targets": [
+    {
+      "name": "local-lvm",
+      "type": "lvmthin",
+      "durable_id": "pve/data",
+      "state": "attached",
+      "reachable": true,
+      "total_bytes": 100000000000,
+      "used_bytes": 42000000000,
+      "avail_bytes": 58000000000,
+      "used_fraction": 0.42,
+      "content": "rootdir,images",
+      "mount_path": "",
+      "backing_device": "",
+      "class_hint": "fast",
+      "role": "",
+      "thin_pool": { "data_used_fraction": 0.42, "metadata_used_fraction": null },
+      "smart": {
+        "health": "UNKNOWN",
+        "temperature_c": null,
+        "power_on_hours": null,
+        "reallocated_sectors": null,
+        "pending_sectors": null,
+        "offline_uncorrectable": null,
+        "critical_warning": null,
+        "media_errors": null,
+        "percentage_used": null
+      }
+    },
+    {
+      "name": "usb-backup",
+      "type": "usb",
+      "durable_id": "uuid:0fc63daf-8483-4772-8e79-3d69d8477de4",
+      "state": "attached",
+      "reachable": true,
+      "total_bytes": 2000000000000,
+      "used_bytes": 500000000000,
+      "avail_bytes": 1500000000000,
+      "used_fraction": 0.25,
+      "content": "backup",
+      "mount_path": "/mnt/usb-backup",
+      "backing_device": "/dev/sdb1",
+      "class_hint": "slow",
+      "role": "",
+      "smart": {
+        "health": "UNKNOWN",
+        "temperature_c": null,
+        "power_on_hours": null,
+        "reallocated_sectors": null,
+        "pending_sectors": null,
+        "offline_uncorrectable": null,
+        "critical_warning": null,
+        "media_errors": null,
+        "percentage_used": null
+      }
+    }
+  ],
   "backups": [],
   "restore_tests": [],
   "pbs_snapshots": [],