fix(sidecars): keep mirrored sidecars when a model root moves

Centralized sidecars were addressed by a hash of the model root's absolute
path, so moving or renaming a root produced a new mirror directory. The
scanner then found no sidecar there, rebuilt default metadata, and silently
lost favorites, notes, tags and usage tips for every model under that root,
leaving the old metadata orphaned on disk.

Mirrors are now addressed by a root identity pinned in
<sidecar_root>/.lm-sidecar-roots.json. The identity starts as the existing
deterministic <basename>-<path digest> -- so pre-existing mirrors keep
resolving even if the map is lost, and a relocated sidecar root keeps its
names -- and is re-anchored to the root's new path when it moves, matched by
basename and recorded sample directories. Ambiguous matches are never
guessed: the mirror is left untouched and reported.

Also:
- drop the <library> path segment (single-library direction); a legacy
  library prefix is only recognised while adopting an existing mirror, which
  also keeps the unreleased centralized layout usable
- surface stranded mirrors in Doctor and in the log instead of silently
  rebuilding sidecars
- keep the default alongside mode untouched: the identity map is loaded and
  reconciled lazily, only while centralized storage is in use

py/services/use_cases/sidecar_migration_use_case.py

@@ -5,7 +5,8 @@ Two storage layouts exist (see :mod:`py.utils.sidecar_paths`):
 - ``alongside``: ``<model_dir>/<name>.metadata.json`` and preview files live
   next to the model file.
 - ``centralized``: the same files live under the configured sidecar root,
-  mirroring the library-relative directory structure.
+  mirroring each model root's directory structure under a per-root identity
+  component (see :func:`py.utils.sidecar_paths.root_mirror_component`).
 
 This use case moves the ``.metadata.json`` sidecar and preview files for every
 known model from one layout to the other. Model files themselves NEVER move.