feat(core): add TrackRequest system for multi-codec/multi-range support

Add TrackRequest dataclass to Service base class that centralizes CLI vcodec/range/best_available params. Services read from self.track_request instead of accessing ctx.parent.params directly.

- Add TrackRequest dataclass with codecs, ranges, best_available fields
- Set self.track_request in Service.__init__() from CLI params
- Add _get_tracks_for_variants() helper for per-codec/range API calls
- Update dl.py to detect migrated vs legacy services automatically
- Handle HYBRID+other ranges (e.g. -r HYBRID,SDR) correctly in dl.py
- Support --best-available with multi-range/codec (skip unavailable)

unshackle/core/utils/click_types.py

@@ -44,6 +44,33 @@ class VideoCodecChoice(click.Choice):
         self.fail(f"'{value}' is not a valid video codec", param, ctx)
 
 
+class MultipleVideoCodecChoice(VideoCodecChoice):
+    """
+    A multiple-value variant of VideoCodecChoice that accepts comma-separated codecs.
+
+    Accepts both enum names and values, e.g.: ``-v hevc,avc`` or ``-v H.264,H.265``
+    """
+
+    name = "multiple_video_codec_choice"
+
+    def convert(
+        self, value: Any, param: Optional[click.Parameter] = None, ctx: Optional[click.Context] = None
+    ) -> list[Any]:
+        if not value:
+            return []
+        if isinstance(value, list):
+            values = value
+        elif isinstance(value, str):
+            values = value.split(",")
+        else:
+            self.fail(f"{value!r} is not a supported value.", param, ctx)
+
+        chosen_values: list[Any] = []
+        for v in values:
+            chosen_values.append(super().convert(v.strip(), param, ctx))
+        return chosen_values
+
+
 class SubtitleCodecChoice(click.Choice):
     """
     A custom Choice type for subtitle codecs that accepts both enum names, values, and common aliases.