refactor(tv_shows): freeze aggregate, builder-only construction, drop ShowTracker fields

The TVShow aggregate is now fully immutable. TVShow, Season and Episode
are @dataclass(frozen=True), children stored as ordered tuples sorted
by number. All construction goes through TVShowBuilder / SeasonBuilder
(new module), which expose from_existing() to seed from a current
frozen aggregate and apply modifications.

ShowTracker-territory fields are stripped from the domain: ShowStatus,
CollectionStatus, expected_seasons/episodes, aired_episodes,
collection_status(), is_complete_series(), missing_episodes(),
is_ongoing(), is_ended(), Season.name, the aired<=expected validation,
and the TMDB status string mapping. These will reappear in a dedicated
ShowTracker layer (to be designed) combining the .alfred sidecar with
live TMDB data.

New SeasonMode enum (PACK / EPISODIC) computed at read time from the
season's structural shape — never stored, the YAML sidecar encodes the
mode via presence/absence of the episodes: block.

Test suite for the domain entirely rewritten to cover frozen invariants,
builder ordering, last-write-wins, from_existing round-trip, and
SeasonMode derivation. Full suite still green (1078 passed).

CHANGELOG.md

@@ -17,6 +17,64 @@ callers).
 
 ### Added
 
+- **`TVShowBuilder` / `SeasonBuilder` — sole construction surface for the
+  TVShow aggregate** (`alfred/domain/tv_shows/builders.py`). The aggregate
+  is now fully frozen; building goes through a mutable scratchpad that
+  emits an immutable `TVShow` via `build()`. Both builders offer a
+  `from_existing()` classmethod to seed from a current frozen aggregate
+  and apply modifications. Episodes are emitted sorted by number within a
+  season, seasons sorted by number within the show.
+- **`SeasonMode` enum** (`PACK` / `EPISODIC`) in
+  `alfred/domain/tv_shows/value_objects.py`. Computed at read time from
+  the season's structural shape (`Season.mode` property): a season with
+  no explicit episodes is `PACK` (a single release covering the whole
+  season), a season with episodes is `EPISODIC` (currently airing, one
+  release per episode). Never stored — the YAML sidecar encodes the
+  mode via the presence/absence of the `episodes:` block.
+
+### Changed
+
+- **TVShow aggregate is now frozen all the way down.** `TVShow`,
+  `Season` and `Episode` are all `@dataclass(frozen=True)`. Children
+  are stored as ordered tuples (`tuple[Season, ...]`,
+  `tuple[Episode, ...]`) sorted by their respective numbers, replacing
+  the previous mutable dicts. Lookup helpers `TVShow.get_season(n)` and
+  `Season.get_episode(n)` traverse the tuple lazily via `next()`. The
+  former `add_episode` / `add_season` mutation methods are gone — all
+  construction goes through `TVShowBuilder` / `SeasonBuilder`.
+
+### Removed
+
+- **ShowTracker-territory fields stripped from the TVShow aggregate.**
+  The aggregate now models only what the `.alfred` sidecar stores
+  (filesystem-observable facts + immutable identity). Dropped from the
+  domain:
+  - `TVShow.status` (`ShowStatus`) and the `ShowStatus` enum entirely,
+    along with its TMDB string mapping (`from_string`).
+  - `TVShow.expected_seasons`, `Season.expected_episodes`,
+    `Season.aired_episodes`, `Season.name`.
+  - `TVShow.collection_status()`, `is_complete_series()`,
+    `missing_episodes()`, `is_ongoing()`, `is_ended()` and the
+    `CollectionStatus` enum.
+  - `Season.is_complete()`, `is_fully_aired()`, `missing_episodes()`
+    and the `aired ≤ expected` validation.
+  - `TVShow.add_episode()` / `TVShow.add_season()` /
+    `Season.add_episode()` — replaced by the builder API.
+  These concerns will reappear in a dedicated `ShowTracker` layer (to
+  be designed) that combines the `.alfred` sidecar with live TMDB data
+  to answer questions like "is this show complete?" or "are new
+  episodes out?". Keeping volatile/derived state out of the aggregate
+  matches the factuel-only philosophy locked in `specs/dot_alfred.md`.
+
+### Internal
+
+- **Test suite rewritten for the new aggregate shape.**
+  `tests/domain/test_tv_shows.py` now covers frozen invariants, builder
+  ordering, last-write-wins on duplicates, `from_existing` round-trip,
+  and `SeasonMode` derivation. `tests/infrastructure/test_filesystem_extras.py`
+  helper simplified (no more `ShowStatus.ENDED` / `expected_seasons` on
+  test shows). 1078 tests still green.
+
 - **Design doc for `.alfred/` sidecar persistence
   (`specs/dot_alfred.md`).** First entry in the new `specs/` directory.
   Specifies a per-show `.alfred/` directory holding a `show.yaml` and