docs(casting): add Chromecast cast test matrix

docs/chromecast-test-matrix.md

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+# Chromecast Cast Test Matrix
+
+Manual verification for the device-profile work. Run each row by casting the
+matching media from the app to a physical Chromecast and recording the result.
+
+**Test device:** ___________________  (model name as reported by the app)
+**App build / commit:** ___________________
+**Date:** ___________________
+
+## How to run
+
+1. Pick a library item matching the row's codec / audio / container.
+2. Cast it. Note whether it direct-plays or transcodes (server logs show
+   `Video is being transcoded` vs `Video is being direct played`).
+3. Record the load result: OK / 2100 / infinite-loading / other.
+
+## Matrix
+
+| # | Video codec | Audio | Container | Approx bitrate | Direct/Transcode | Result | Notes |
+|---|---|---|---|---|---|---|---|
+| 1 | H.264 1080p | AAC stereo | MP4 | ~4 Mb/s | | | |
+| 2 | H.264 1080p | AAC stereo | MKV | ~8 Mb/s | | | |
+| 3 | H.264 1080p | AAC 5.1 | MKV | ~8 Mb/s | | | |
+| 4 | H.264 1080p | AC3 5.1 | MKV | ~10 Mb/s | | | |
+| 5 | H.264 1080p | DTS 5.1 | MKV | ~12 Mb/s | | | |
+| 6 | H.264 1080p | TrueHD 7.1 | MKV | ~20 Mb/s | | | |
+| 7 | H.264 1080p | AAC stereo | MP4 | ~16 Mb/s | | | |
+| 8 | H.264 1080p | AAC stereo | MP4 | source-max | | | |
+| 9 | HEVC 8-bit | AAC stereo | MKV | ~10 Mb/s | | | |
+| 10 | HEVC 10-bit | AAC stereo | MKV | ~15 Mb/s | | | |
+
+## Outcome
+
+- Highest video bitrate that loads reliably on the test device: ___________
+  -> update `CONSERVATIVE_CAPABILITIES.maxVideoBitrate` in
+  `utils/casting/capabilities.ts` accordingly.
+- Confirmed cause of issue #1423 (<= 2 Mb/s): ___________
+- Confirmed cause of the 5.1 crash (#1085): ___________
+- Cases where downgrade-on-failure retry rescued playback: ___________