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>

curateipsum/fs.py

@@ -561,6 +561,11 @@ def nest_hardlink(src_dir: str, src_relpath: str, dst_dir: str):
     """
     _lg.debug("Nested hardlinking: %s%s%s -> %s",
               src_dir, os.path.sep, src_relpath, dst_dir)
+    if os.path.isabs(src_relpath) or os.path.pardir in src_relpath.split(
+            os.path.sep):
+        raise RuntimeError(
+            "Refusing non-normalized relative path: %s" % src_relpath)
+
     src_dir_abs = os.path.abspath(src_dir)
     src_full_path = os.path.join(src_dir_abs, src_relpath)
     dst_dir_abs = os.path.abspath(dst_dir)