feat(dl): add --audio-description-only and --forced-subs-only

Each keeps one kind of track inside its track type. A run of more than one title skips a title that has no such track. The REST API rejects a non-boolean value for a boolean dl param, and the DASH parser reads audio description from Role elements.

docs/guide/cli-reference.md

@@ -102,6 +102,7 @@ unshackle dl [OPTIONS] SERVICE [SERVICE ARGS...]
 | `--require-video` | - | Video langs that must exist. The title fails even with `--best-available`. Does not select tracks. |
 | `--require-subs` | - | Subtitle langs that must exist. Fails the title if one is missing. `--s-lang` still decides what to keep. |
 | `-fs`, `--forced-subs` | off | Include forced subtitle tracks. |
+| `-fso`, `--forced-subs-only` | off | Keep only forced subtitle tracks; drop the other subtitles. Implies `-fs`. |
 | `-fsl`, `--forced-s-lang` | none | Language(s) wanted for forced subtitles; implies `-fs`. A `-` prefix excludes. |
 | `--exact-lang` | off | Exact matching only: `-l es-419` matches `es-419`, not `es-ES`. Applies to selection and to sort order. |
 | `--sub-format` | - | Output subtitle format (`SRT`/`srt`, `VTT`/`webvtt`, `ASS`/`ssa`, `TTML`, `SMI`, ...), or `original` to keep the source format. |
@@ -139,6 +140,7 @@ Keep only certain track types, or skip certain track types.
 | Flag | Description |
 |---|---|
 | `-ad`, `--audio-description` | Include descriptive (audio-description) tracks. |
+| `-ado`, `--audio-description-only` | Keep only descriptive (audio-description) tracks; drop the standard audio. Does not need `-ad`. |
 | `--skip-subtitle-errors` | Skip a failed subtitle instead of aborting the title. Video/audio failures remain fatal. |
 
 ### Output, muxing & files

docs/guide/downloading.md

@@ -416,6 +416,8 @@ video track counts. `--no-audio` and `--no-video` drop the matching requirement.
   `-fs`). Works independently of `--s-lang`, so `-sl all -fsl en` grabs every full
   subtitle but only the English forced track. It accepts exclusions too: `-fsl all,-es`
   keeps every forced subtitle except the Spanish one, and `-fsl -es` means the same.
+- `-fso` / `--forced-subs-only`: keep only the forced subtitle tracks and drop every
+  other subtitle (implies `-fs`). `--s-lang` and `-fsl` still select the languages.
 - `--sub-format`: set the output subtitle format, converting only when necessary.
   Accepts codec names/values and common aliases (`srt`, `vtt`, `ass`, `ssa`, `ttml`,
   and the other codec aliases), or the literal `original` to keep the source format.
@@ -637,9 +639,29 @@ Additional track-type flags:
 
 - `-ad` / `--audio-description`: include descriptive (audio-description) tracks, which
   unshackle drops by default.
+- `-ado` / `--audio-description-only`: keep only the descriptive tracks and drop the
+  standard audio. It does not need `-ad`. Add `-A` to download no video or subtitles.
 - `--skip-subtitle-errors`: if a subtitle fails to download, skip it and continue rather
   than aborting the whole title. Video and audio failures remain fatal.
 
+!!! note "The `-only` kind flags"
+    `-ado` and `-fso` each keep one kind of track inside its track type. They do not drop
+    the other track types: `-ado` still downloads the video, and `-fso` still downloads
+    video and audio. Add `-A` or `-S` for that.
+
+    - The language flags still apply. With the default `-l orig`, `-ado` fails when the
+      descriptive track is not in the original language. Select the language with `-al`.
+    - A flag that drops the same tracks is an error, for example `-ado` with `--no-audio`
+      or `-fso` with `-A`.
+    - A title with no track of that kind stops a run of one title. A run of more than one
+      title skips that title with a warning and continues.
+    - unshackle reads the kind from the manifest or from the service. A track that
+      neither of them marks as descriptive or forced does not match.
+
+```shell title="Audio description only, in English"
+unshackle dl -A -ado -al en EXAMPLE 81234567
+```
+
 ```shell title="Subtitles only"
 unshackle dl -S -sl en EXAMPLE 81234567
 ```

docs/guide/subtitles.md

@@ -74,6 +74,7 @@ Subtitle selection happens through `dl` flags. The most important is language se
 | `-sl`, `--s-lang` | Language(s) wanted for subtitles. Defaults to `all`. A `-` prefix excludes, e.g. `all,-es`. |
 | `--require-subs` | Require these languages to exist; if present, download **all** subtitles. Cannot be combined with `--s-lang`. |
 | `-fs`, `--forced-subs` | Include forced subtitle tracks (excluded by default). |
+| `-fso`, `--forced-subs-only` | Keep only forced subtitle tracks and drop the other subtitles; implies `-fs`. |
 | `-fsl`, `--forced-s-lang` | Language(s) wanted for forced subtitles; implies `-fs`. A `-` prefix excludes. |
 | `--exact-lang` | Exact language matching, with no regional variants. |
 | `-S`, `--subs-only` | Download only subtitle tracks. |
@@ -138,6 +139,17 @@ If you pass both flags, `-fsl` wins and keeps only forced tracks in its language
 It also understands `orig` (the title's original language) and `all` (every forced
 track, same as plain `-fs`).
 
+To download the forced tracks and no other subtitle, use `-fso` / `--forced-subs-only`.
+It implies `-fs`, and `--s-lang` and `-fsl` still select the languages:
+
+```shell title="Only the English forced track"
+unshackle dl --s-lang en --forced-subs-only EXAMPLE 81234567
+```
+
+`-fso` does not drop video or audio. Add `-S` for a subtitle-only download. A title with
+no forced track stops a run of one title. A run of more than one title skips that title
+with a warning.
+
 ## Track types: forced, SDH, and CC
 
 Every subtitle track carries flags describing what kind of captions it holds. These