Validate topology and destructive path boundaries (#7)

* Validate topology and destructive path boundaries

- Canonicalize sources/backups_dir and reject overlap in either direction
- Reject duplicate sources and destination basename collisions
- Reject non-normalized relative paths in nest_hardlink
- Give each backup repository a persistent identity, recorded in every
  snapshot's completion manifest
- Verify repo identity, directory type (no symlinks), containment, and
  device before any recursive snapshot/staging-dir deletion, refusing to
  cross mount points

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Address Copilot review: root-path check, symlink/TOCTOU-safe writes, legacy marker adoption

- _is_same_or_within: drop the redundant, root-mishandling startswith
  check; commonpath() alone is already component-aware
- _ensure_repo_id/_write_manifest: write via temp-file + atomic rename
  (O_NOFOLLOW, O_EXCL on the temp file) instead of open+write in place,
  so writes can never go through a pre-existing symlink and are never
  left half-written
- _verify_safe_to_delete: distinguish a legacy empty marker (pre-dates
  the manifest format, can only be a pre-migration artifact) from a
  non-empty-but-invalid one (corrupt or forged); adopt the former into
  the repository instead of refusing to delete it forever

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

tests/conftest.py

@@ -106,7 +106,10 @@ def check_backup_not_empty():
 def check_backups(backup_dir, check_backup_not_empty):
     """Helper to verify expected backups exist."""
     def _check(expected_backups):
-        backups_list = os.listdir(str(backup_dir))
+        backups_list = [
+            name for name in os.listdir(str(backup_dir))
+            if name != bk.REPO_ID_FILE
+        ]
         assert sorted(b.name for b in expected_backups) == sorted(backups_list)
         for b in expected_backups:
             assert check_backup_not_empty(b)