feat(tracks): add native track names with muxing.track_names

Set muxing.track_names to native to name an unnamed track in its own language (中文（繁體）, Português (Portugal)) in place of English. The same script and region rules apply. --list, the muxed names and the REST API track_name follow the option. A test fixture resets the muxing options, so a local unshackle.yaml cannot change test results.

docs/dev/rest-api/endpoints.md

@@ -377,7 +377,7 @@ The server sorts tracks by bitrate, descending. Episodes the server cannot get g
 
 The `drm` array on a track is a list of `{type, pssh?, kids?, content_keys?, license_url?}` objects.
 
-Every track carries the same fields as a local track: `is_original_lang`, `name`, `needs_repack`, `edition`, and `data`. `name` is the name the service set, or null. `track_name` is the name `--list` shows and the mux writes to an audio or subtitle track: `name` when the service set one, else the English name of the language, with the SDH, CC, Forced or Descriptive label. `bitrate` is in kb/s, and `bitrate_bps` gives the exact bits/s. `data` holds the part of the service-set `track.data` that fits in JSON, without the `hls`, `dash`, and `ism` parser keys. Audio keeps `is_original` and `atmos` for clients that read them, and adds the exact `joc` object count.
+Every track carries the same fields as a local track: `is_original_lang`, `name`, `needs_repack`, `edition`, and `data`. `name` is the name the service set, or null. `track_name` is the name `--list` shows and the mux writes to an audio or subtitle track: `name` when the service set one, else the name of the language (English, or in the language itself when the server sets `muxing.track_names: native`), with the SDH, CC, Forced or Descriptive label. `bitrate` is in kb/s, and `bitrate_bps` gives the exact bits/s. `data` holds the part of the service-set `track.data` that fits in JSON, without the `hls`, `dash`, and `ism` parser keys. Audio keeps `is_original` and `atmos` for clients that read them, and adds the exact `joc` object count.
 
 | Status | Error code | Meaning |
 | --- | --- | --- |

docs/guide/output-and-naming.md

@@ -305,6 +305,29 @@ A tag without a region, such as `zh`, gets no script because the region is what
 unshackle writes audio tags as the service gives them, because a script applies only to written text.
 Track selection, `--list` tags and file names use the service tag.
 
+Set `muxing.track_names` to `native` to name each track in its own language. The same rules apply
+to the script and the region:
+
+```yaml title="unshackle.yaml"
+muxing:
+  track_names: native
+```
+
+| Service tag | Track name |
+|-------------|------------|
+| `zh-TW` subtitle | 中文（繁體） |
+| `zh-HK` subtitle | 中文（繁體，香港） |
+| `pt-PT` | Português (Portugal) |
+| `es-419` | Español (Latinoamérica) |
+| `ja-JP` | 日本語 |
+
+The option also changes the names in `--list` and in the `track_name` field of the REST API.
+
+!!! note "Players that show the language next to the name"
+    Some players, such as Jellyfin, show the track name and the language name together. They leave
+    out the language when the name already contains it, so an English name shows once. A native name
+    such as `日本語` shows next to `Japanese` in an English interface.
+
 ### DTS:X Profile 2 output
 
 A title whose audio holds a `dtsx` or `dtsy` sample entry (DTS:X Profile 2, DTS-UHD) muxes

docs/reference/configuration/download.md

@@ -288,6 +288,7 @@ Matroska (MKV) muxing options.
 | `default_language` | dict | `{}` | Force which language is flagged *default* per track type, e.g. `{audio: en, subtitle: en}`. A tag with a region, such as `en-US`, matches only that variant. |
 | `merge_audio` | bool | `true` | Merge all audio into one file. `--split-audio` on the CLI flips this off. |
 | `merge_video` | bool | `false` | Merge video tracks that share height, range, and codec into one file, so only language varies inside a file. `--merge-video` on the CLI flips this on. |
+| `track_names` | str | `english` | The language of the name unshackle gives a track that the service left unnamed: `english` (`Chinese (Traditional)`) or `native`, the name in the language itself (`中文（繁體）`). Read [Track names and language tags](../../guide/output-and-naming.md#track-names-and-language-tags). |
 | `concurrency` | int | `4` | How many mkvmerge processes run at the same time when a title muxes more than one output file (one per video variant or audio codec). Each process uses one CPU core, so this shortens the mux step on an SSD. Set it to `1` on a hard disk, where parallel writes are slower. |
 
 ```yaml