v0.6.0-rc1: slice 6 Phase A — backup + the self-restore-test (local target)

The guest-level backup layer + the journaled self-restore-test (restore→boot→verify→
teardown) that closes "a backup you haven't restored isn't a backup". All benign
(reuses the slice-4 classifier/gate/journal; no new destructive class/crypto). Local
target only; PBS = Phase B. Restore to a NEW guest only. Backups crash-consistent.

- proxmox: DestroyLXC, VzdumpOptions.Notes (notes-template), LatestBackupVolID.
- reconcile: Engine.RunRestoreTest (journal Scratch entry BEFORE mutation; net link-down
  pre-boot; defer teardown always; benign gated destroy) + Recover extended to reap a
  leaked scratch guest (Scratch flag, special-cased before the UPID path; idempotent).
- internal/backup: runner (vzdump + archive resolve + bulk-gap = backup!=1) + cadence
  scheduler (4th daemon goroutine, default 24h) + in-memory report store.
- hub: Backup/RestoreTest filled; collector seams; cross-repo golden byte-identical +
  bidirectional key-set tests; hub handler logs a FAILED restore-test prominently.
- config BackupConfig (band 990000-990009 default); --selftest=backup / restore-test.

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

internal/reconcile/journal.go

@@ -45,7 +45,13 @@ type JournalEntry struct {
 	UPID     string            `json:"upid,omitempty"`
 	State    OpState           `json:"state"`
 	IdempKey string            `json:"idemp_key,omitempty"`
-	At       time.Time         `json:"at"`
+	// Scratch marks an entry that OWNS an agent-tagged scratch guest at VMID (slice 6
+	// restore-test). While such an entry is in-flight, the load-bearing invariant is "the
+	// scratch guest may exist and MUST be destroyed" — so Recover resolves it by ensuring
+	// VMID is gone (a benign teardown), NOT by re-checking any sub-task UPID. The entry is
+	// terminal only after teardown. See recover.go.
+	Scratch bool      `json:"scratch,omitempty"`
+	At      time.Time `json:"at"`
 }
 
 // Journal is the durable operation log + idempotency store. It mirrors