feat(tracks): download and mux stereoscopic 3D (MV-HEVC) video to MP4

Mark 3D from REQ-VIDEO-LAYOUT, DV profile 20 codecs or an lhvC box; mux with MP4Box, decrypt with mp4decrypt, skip FFmpeg repack, sample cut and hybrid; add {3d} and env MP4Box.

docs/dev/rest-api/endpoints.md

@@ -339,7 +339,7 @@ When you give both `season` and `episode`, the server combines them into a `"{se
           "bitrate": 8000, "bitrate_bps": 8000000, "width": 3840, "height": 2160,
           "resolution": "2160p", "fps": "24", "range": "HDR10",
           "range_display": "HDR", "scan_type": null, "closed_captions": [],
-          "dv_compatible_bitstream": false, "hdr10plus": false, "hdr10_base": false, "language": "en",
+          "dv_compatible_bitstream": false, "hdr10plus": false, "hdr10_base": false, "stereo": false, "language": "en",
           "is_original_lang": true,
           "name": null, "track_name": "English", "needs_repack": false, "edition": [], "note": null, "data": null,
           "drm": [], "descriptor": "DASH"

docs/dev/service-architecture.md

@@ -454,6 +454,7 @@ Two DV mechanisms sit on top of the track model. **DV fixup** re-exposes DV sign
 HLS and DASH streams that carry a valid DV RPU but advertise plain HEVC, with DV only in the
 supplemental codecs (`dv_compatible_bitstream`). `--list` shows such a track as `HDR10+DV` or `HDR10P+DV`.
 A service sets `hdr10_base` on a DV track that has an HDR10 base layer (DV profile 8.1), and `--list` shows it as `DV+HDR10`.
+The HLS and DASH parsers set `stereo` on a stereoscopic 3D (MV-HEVC) track: an HLS `REQ-VIDEO-LAYOUT` that names `CH-STEREO`, or a DV profile 20 codec. `--list` shows it as `3D`.
 **Hybrid** processing (`Video.Range.HYBRID`) selects an HDR10(+) base and a DV donor,
 extracts the RPU with `dovi_tool`, and injects it onto the base layer, to make a single
 track that carries both HDR10 metadata and the DV RPU. Both are automatic once the