feat(library): add rescan_show orchestrator + walker (Step 4)

Step 4 of specs/dot_alfred.md — rebuild a TVShow aggregate from disk
by reusing the existing release pipeline (inspect_release) on every
video file in a show folder, then persist via the .alfred repository.

- alfred/application/library/walker.py — pure structural walk
  (season folders detected via \bS\d{1,2}\b regex, video files
  filtered against kb.video_extensions, no recursion).
- alfred/application/library/rescan.py — orchestrator that ingests
  each season folder, infers PACK vs EPISODIC from on-disk file
  count + parser output, and assembles via TVShowBuilder. Episode
  paths stored relative to show_root. Logs + skips corrupt input
  (no season parsed, mixed season numbers, unparseable episodes).
- Season now inherits MediaWithTracks: PACK seasons carry
  season-level audio_tracks / subtitle_tracks; EPISODIC seasons
  leave them empty (tracks live per-episode). SeasonBuilder gains
  set_audio_tracks / set_subtitle_tracks; bridge writes/reads them
  in the PACK branch via shared _synth_* helpers.

Out of scope, tracked as tech debt: adjacent .srt capture, multi-
episode (episode_end), TMDB-driven PACK detection (the current
heuristic '1 file == PACK' is a placeholder until ShowTracker lands).

18 new tests (11 walker + 7 rescan integration) on tmp_path with
the Foundation layout. Full suite: 1149 passed.

CHANGELOG.md

@@ -17,6 +17,50 @@ callers).
 
 ### Added
 
+- **`rescan_show` orchestrator
+  (`alfred/application/library/rescan.py`).** Step 4 of the
+  `specs/dot_alfred.md` plan. Walks an Alfred-managed show folder,
+  runs the existing `inspect_release` pipeline on every video file it
+  finds, and assembles a frozen `TVShow` aggregate persisted via the
+  injected `TVShowRepository`. Reuses the release parser + ffprobe
+  path verbatim — no duplicated parse/probe logic at the library
+  layer. PACK vs EPISODIC inferred per season folder from the
+  on-disk file count + parser output: a single video whose name
+  carries no `Exx` token becomes a PACK season (tracks lifted to the
+  season-level `audio_tracks` / `subtitle_tracks`), anything else
+  becomes EPISODIC (one `Episode` per file). Episode paths are
+  stored relative to the show root for portability. Files that fail
+  to parse a season/episode number, or seasons with mixed numbers,
+  are logged and skipped — the orchestrator never raises. Embedded
+  subtitle tracks are captured from `ffprobe`; adjacent `.srt`
+  files, multi-episode entries (`S01E01E02`), and TMDB-driven PACK
+  detection are tracked as tech debt for a dedicated subtitles /
+  ShowTracker session. 7 integration tests on `tmp_path` with the
+  Foundation layout (S01 EPISODIC + S02 PACK) cover the round-trip
+  through the real `.alfred` repository.
+- **Show tree walker (`alfred/application/library/walker.py`).**
+  Step 4a foundation. `walk_show(show_root, scanner, kb)` returns a
+  `ShowTree(show_root, season_folders=tuple[SeasonFolder, ...])` —
+  pure structural snapshot, no parsing, no probing. Season folders
+  are detected by a `\bS\d{1,2}\b` token anywhere in the directory
+  name (release-style naming, no Plex `Season 01` / `Specials`
+  conventions). Video files are filtered against
+  `kb.video_extensions`; no recursion into sub-sub-folders. 11 unit
+  tests on `tmp_path` cover detection (case-insensitive, in-word
+  rejection), filtering (subs, NFO, sample files), and edge cases
+  (empty / missing show root).
+- **Season-level audio/subtitle tracks
+  (`alfred/domain/tv_shows/entities.py`,
+  `alfred/domain/tv_shows/builders.py`).** `Season` now inherits
+  from `MediaWithTracks` and carries `audio_tracks` /
+  `subtitle_tracks` tuples (empty by default). Populated only in
+  PACK mode (the single release covering the whole season); empty in
+  EPISODIC mode where tracks live per-episode. `SeasonBuilder`
+  gains `set_audio_tracks()` / `set_subtitle_tracks()` and forwards
+  them through `from_existing()`. The bridge writes / reads them in
+  the PACK branch via shared `_synth_audio_tracks` /
+  `_synth_subtitle_tracks` helpers used for episodes too.
+
 - **`DotAlfredTVShowRepository` — filesystem-backed implementation of
   the `TVShowRepository` port
   (`alfred/infrastructure/persistence/dot_alfred/repository.py`).**