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/hub/testdata/host-report.golden.json

@@ -86,8 +86,31 @@
       }
     }
   ],
-  "backups": [],
-  "restore_tests": [],
+  "backups": [
+    {
+      "target_id": "local",
+      "vmid": 9001,
+      "archive": "local:backup/vzdump-lxc-9001-2026_06_09-11_00_00.tar.zst",
+      "mode": "snapshot",
+      "crash_consistent": true,
+      "size_bytes": 524288000,
+      "success": true,
+      "started_at": "2026-06-09T11:00:00Z",
+      "duration_seconds": 42.5,
+      "uncovered_volumes": ["/mnt/bulk"]
+    }
+  ],
+  "restore_tests": [
+    {
+      "source_archive": "local:backup/vzdump-lxc-9001-2026_06_09-11_00_00.tar.zst",
+      "source_tier": "local",
+      "scratch_vmid": 990000,
+      "pass": true,
+      "verified": "boot+running",
+      "tested_at": "2026-06-09T11:05:00Z",
+      "duration_seconds": 38.2
+    }
+  ],
   "pbs_snapshots": [],
   "cloudflared": { "status": "active" },
   "audit_tail": []