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.

tests/application/library/test_walker.py

@@ -0,0 +1,128 @@
+"""Tests for the show-tree walker."""
+
+from __future__ import annotations
+
+from alfred.application.library.walker import walk_show
+from alfred.infrastructure.filesystem.scanner import PathlibFilesystemScanner
+from alfred.infrastructure.knowledge.release_kb import YamlReleaseKnowledge
+
+_KB = YamlReleaseKnowledge()
+_SCANNER = PathlibFilesystemScanner()
+
+
+def _episode(dir_path, name: str) -> None:
+    (dir_path / name).write_bytes(b"")
+
+
+class TestSeasonFolderDetection:
+    def test_keeps_folders_with_season_token(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        (show / "Foundation.S01.1080p.WEB-DL.x265-GROUP").mkdir()
+        (show / "Foundation.S02.Complete.1080p.WEB-DL-GROUP").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        names = sorted(s.season_dir.name for s in tree.season_folders)
+        assert names == [
+            "Foundation.S01.1080p.WEB-DL.x265-GROUP",
+            "Foundation.S02.Complete.1080p.WEB-DL-GROUP",
+        ]
+
+    def test_skips_folders_without_season_token(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        (show / "Sample").mkdir()
+        (show / "Soundtrack").mkdir()
+        (show / "Foundation.S01.WEB").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        assert [s.season_dir.name for s in tree.season_folders] == [
+            "Foundation.S01.WEB"
+        ]
+
+    def test_season_token_is_case_insensitive(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        (show / "Foundation.s01.WEB").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        assert len(tree.season_folders) == 1
+
+    def test_season_token_rejects_in_word(self, tmp_path):
+        # "Season01" without separator must not match (we want a real
+        # Sxx token, not any letter S followed by digits).
+        show = tmp_path / "WeirdShow"
+        show.mkdir()
+        (show / "Pass100").mkdir()
+        (show / "ABS01XYZ").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        assert tree.season_folders == ()
+
+    def test_ignores_files_at_show_root(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        (show / "Foundation.S01.mkv").write_bytes(b"")  # file, not a folder
+        (show / "Foundation.S01.WEB").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        assert [s.season_dir.name for s in tree.season_folders] == [
+            "Foundation.S01.WEB"
+        ]
+
+
+class TestVideoFileCollection:
+    def test_keeps_only_video_extensions(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        season = show / "Foundation.S01.WEB"
+        season.mkdir()
+        _episode(season, "Foundation.S01E01.mkv")
+        _episode(season, "Foundation.S01E02.mp4")
+        _episode(season, "Foundation.S01E01.eng.srt")  # not a video
+        _episode(season, "info.nfo")  # not a video
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        (folder,) = tree.season_folders
+        suffixes = sorted(p.suffix for p in folder.video_files)
+        assert suffixes == [".mkv", ".mp4"]
+
+    def test_empty_season_folder_surfaces(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        (show / "Foundation.S01.WEB").mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        (folder,) = tree.season_folders
+        assert folder.video_files == ()
+
+    def test_single_pack_video_is_kept(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        season = show / "Foundation.S01.Complete.WEB"
+        season.mkdir()
+        _episode(season, "Foundation.S01.Complete.WEB.mkv")
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        (folder,) = tree.season_folders
+        assert [p.name for p in folder.video_files] == [
+            "Foundation.S01.Complete.WEB.mkv"
+        ]
+
+    def test_does_not_recurse_into_sub_subfolders(self, tmp_path):
+        show = tmp_path / "Foundation"
+        show.mkdir()
+        season = show / "Foundation.S01.WEB"
+        season.mkdir()
+        nested = season / "Extras"
+        nested.mkdir()
+        _episode(nested, "Foundation.S01E01.mkv")
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        (folder,) = tree.season_folders
+        assert folder.video_files == ()
+
+
+class TestEdgeCases:
+    def test_empty_show_root(self, tmp_path):
+        show = tmp_path / "Empty"
+        show.mkdir()
+        tree = walk_show(show, scanner=_SCANNER, kb=_KB)
+        assert tree.show_root == show
+        assert tree.season_folders == ()
+
+    def test_missing_show_root_returns_empty_tree(self, tmp_path):
+        # Scanner returns [] for non-existent paths; walker must not raise.
+        tree = walk_show(tmp_path / "Nope", scanner=_SCANNER, kb=_KB)
+        assert tree.season_folders == ()