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/proxmox/types.go

@@ -138,6 +138,12 @@ func (g *GuestConfig) prefixed(prefix string) map[string]string {
 
 // Storage is one entry of GET /storage (cluster) and GET /nodes/{node}/storage
 // (the latter adds usage fields). Unused fields stay zero.
+//
+// The lower block (Server/Export/Share/Datastore/Fingerprint/VGName/ThinPool) are the
+// type-specific config fields the cluster /storage definition carries; they are the
+// source for slice-5's deterministic durable_id derivation (server:export for NFS/CIFS,
+// repo+fingerprint for PBS, vg/pool for lvmthin). Additive parse-only fields — decoding
+// ignores unknown keys, so a storage type that lacks one simply leaves it zero.
 type Storage struct {
 	Storage      string  `json:"storage"`
 	Type         string  `json:"type"`    // "dir" | "lvmthin" | "nfs" | "cifs" | "pbs"
@@ -150,6 +156,15 @@ type Storage struct {
 	Enabled      int     `json:"enabled,omitempty"`
 	Shared       int     `json:"shared,omitempty"`
 	UsedFraction float64 `json:"used_fraction,omitempty"`
+
+	// Type-specific config (durable_id sources).
+	Server      string `json:"server,omitempty"`      // nfs/cifs/pbs server host
+	Export      string `json:"export,omitempty"`      // nfs export path
+	Share       string `json:"share,omitempty"`       // cifs share name
+	Datastore   string `json:"datastore,omitempty"`   // pbs datastore name
+	Fingerprint string `json:"fingerprint,omitempty"` // pbs server cert fingerprint
+	VGName      string `json:"vgname,omitempty"`      // lvm/lvmthin volume group
+	ThinPool    string `json:"thinpool,omitempty"`    // lvmthin pool LV name
 }
 
 // StorageContent is one entry of GET /nodes/{node}/storage/{store}/content