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>

IMPROVEMENTS.md

@@ -56,15 +56,15 @@ Acceptance criteria:
 
 ### Validate topology and destructive path boundaries
 
-- [ ] Canonicalize source and backup paths and reject every overlap.
-- [ ] Reject duplicate sources and destination basename collisions, or require
+- [x] Canonicalize source and backup paths and reject every overlap.
+- [x] Reject duplicate sources and destination basename collisions, or require
   explicit source labels.
-- [ ] Require normalized relative paths for nested operations.
-- [ ] Verify `commonpath` before every write, rename, or deletion.
-- [ ] Give each backup repository an identity recorded in its manifest.
-- [ ] Validate repository identity, snapshot type, marker, and device before
+- [x] Require normalized relative paths for nested operations.
+- [x] Verify `commonpath` before every write, rename, or deletion.
+- [x] Give each backup repository an identity recorded in its manifest.
+- [x] Validate repository identity, snapshot type, marker, and device before
   recursive deletion.
-- [ ] Refuse to cross mount points during deletion.
+- [x] Refuse to cross mount points during deletion.
 
 Acceptance criteria: