feat(titles): add optional Episode.part, selected with -w s1e1.2

Opt-in and additive: an episode without a part behaves exactly as before.

docs/dev/creating-a-service.md

@@ -304,11 +304,14 @@ Common constructor fields:
 
     ```python
     Episode(id_, service, title, season, number, name=None, year=None,
-            language=None, data=None, description=None, air_date=None)
+            language=None, data=None, description=None, air_date=None,
+            part=None)
     ```
 
     Pass `air_date` for daily/sports content to name by date instead of
-    `SxxExx`. Return a `Series([...])` of episodes.
+    `SxxExx`. Return a `Series([...])` of episodes. Pass `part` only when the
+    service splits one episode into several videos, see
+    [Split episodes](#split-episodes).
 
 === "Song"
 
@@ -328,6 +331,52 @@ Common constructor fields:
     framework path to reuse cached results and gracefully fall back to stale data
     when the API errors. It also applies the per-service `title_map` config.
 
+#### Split episodes {#split-episodes}
+
+Some services split one logical episode into several separately playable videos, for
+example a variety show that ships episode 1 as *Episode 1 (Part 1)*, *(Part 2)* and
+*(Part 3)*. `Episode.number` stays an `int`. The optional `part` carries the index.
+
+```python
+Episode(..., season=1, number=1, part=1, name="The Reckoning")
+Episode(..., season=1, number=1, part=2, name="The Reckoning")
+```
+
+1. `part` counts from 1, contiguous. `part=0` is rejected, and `-w` can only address
+   parts up to 99.
+2. Every part of one episode shares the same `season` and `number`.
+3. Each part is a separate `Episode` with its own unique `id_`.
+4. Unsplit episode: leave `part` unset. Never pass `part=1` for a whole episode.
+5. Do not put the part in `name` or `number`. Pass a clean `name` (or `None`) and set
+   `part`.
+6. `part` is a split *episode*, not a split *season*. A service listing *Season 1 Part 2*
+   describes a group of episodes. Keep numbering those in sequence, the way the shipped
+   services do.
+
+`part` accepts an `int` or a digit string. A value that is not an integer raises
+`TypeError`. A value of `0` or less raises `ValueError`.
+
+!!! warning "The framework never infers a part from the episode name"
+    Rule 6 is the reason. Shipped services already use the word "Part" to mean a *season*
+    part, so name-sniffing would misread every one of them. The part has to arrive
+    structurally, as the `part` argument.
+
+!!! tip "Clean the part out of the name"
+    `Episode` discards any `name` that starts with `Episode <number>`, so a raw
+    `"Episode 1 (Part 2): The Reckoning"` leaves you with no episode name at all. Strip
+    the prefix before you pass it, or pass `None`.
+
+Once `part` is set, the filename gains a part token (`S01E01.Part.2`, see
+[Output & naming](../guide/output-and-naming.md#split-episodes)), `-w S01E01.2` selects a
+single part while a bare `-w S01E01` selects them all, and the `--list-titles` tree labels
+the episode `01.2`. The REST API reports it as an extra `part` key on the serialized
+title, described in [Endpoints](rest-api/endpoints.md#post-apilist-titles).
+
+!!! note "Opt-in and additive"
+    Leaving `part` unset costs you nothing: filenames, sort order, `-w` matching and the
+    API JSON are identical to a plain episode, so a service that does not split episodes
+    has nothing to adopt.
+
 ### `get_tracks`: required
 
 Given one title, return a `Tracks` object holding `Video`, `Audio`, and

docs/dev/rest-api/endpoints.md

@@ -223,6 +223,12 @@ Service-specific CLI options may also be passed as additional keys.
 
 Each serialized title carries `type` (`"episode"`, `"movie"`, or `"other"`), `name`, `id`, `language`, `description`, `date`, and `cover_url`. Episodes and movies add `year`; episodes additionally add `series_title`, `season`, and `number`.
 
+!!! info "`part` on a split episode"
+    A service that splits one episode into several separately playable videos reports each
+    one as its own episode. Those titles share a `season` and `number` and add a `part`
+    integer counting from 1. No other title carries the key, so a client that ignores it
+    is unaffected. See [Split episodes](../creating-a-service.md#split-episodes).
+
 | Status | Error code | Meaning |
 | --- | --- | --- |
 | `200` | - | Titles returned. |
@@ -248,9 +254,10 @@ List the video, audio, and subtitle tracks for a title. For series, you can scop
 | `wanted` | string | no | Episode/season range (e.g. `"S01E01-S01E03"`). |
 | `season` | int/string | no | Season number (combined with `episode`). |
 | `episode` | int/string | no | Episode number (combined with `season`). |
+| `part` | int/string | no | Part index of a split episode (combined with `season` and `episode`). |
 | `profile`, `proxy`, `no_proxy`, `cdm_type` | - | no | As on `list-titles`. |
 
-When both `season` and `episode` are given, they are combined into a `"{season}x{episode}"` selector.
+When both `season` and `episode` are given, they are combined into a `"{season}x{episode}"` selector. Adding `part` narrows that to `"{season}x{episode}.{part}"`, one part of a [split episode](../creating-a-service.md#split-episodes); without it you get every part of the episode. `part` is read only when `season` and `episode` are both given, and `wanted` takes precedence over all three. To scope a part by range, put it in `wanted` instead (`"S01E01.2"`).
 
 === "Single title / movie: Response `200`"
 
@@ -361,7 +368,7 @@ Create a download job. Requires `service` and `title_id`; every other field is a
 | `range` | string[] | `["SDR"]` | Dynamic range(s). |
 | `channels` | number | `null` | Audio channel count. |
 | `no_atmos` | boolean | `false` | Exclude Atmos tracks. |
-| `wanted` | string[] | `[]` | Episode/season selectors. |
+| `wanted` | string[] | `[]` | Episode/season selectors. Accepts the part form, `"S01E01.2"`. |
 | `latest_episode` | boolean | `false` | Only the newest episode. |
 | `lang` / `v_lang` / `a_lang` / `s_lang` | string[] | `["orig"]` / `[]` / `[]` / `["all"]` | Language filters. |
 | `require_subs` | string[] | `[]` | Required subtitle languages. |