docs(player): allocate UR-074/DR-162 for the streaming bitrate cap

The feature shipped tagged against DR-160, which a parallel session had
claimed for picture-in-picture in the meantime. Renumbered to DR-162
across the Rust and frontend TRACES comments (the PiP tags in
VideoPlayer.svelte, pictureInPicture.ts and nativeVideo.ts keep DR-160)
and regenerated bindings.ts.

Adds the requirement rows the tags point at: UR-074 for the user need, and
DR-162 covering why the cap has to reach the PlaybackInfo negotiation and
not only the transcode URL, why the ceiling is process-wide, and why the
Settings default persists while the in-player override does not. Notes
that this gives UR-070 its resume-at-the-same-point mechanism while the
server-offered rendition list that requirement also asks for stays
proposed. UT-156/157 record what the tests pin.

docs/specs/streaming-bitrate-cap.md carries the layer assignment — the
step definitions, the video/audio split, the resolution pairing and the
reload decision are all Rust; the frontend holds a serde token and the
labels it was handed.

TRACES: UR-074 | DR-162 | UT-156, UT-157

src/lib/components/player/VideoPlayer.scrubRegression.test.ts

@@ -82,6 +82,10 @@ vi.mock("$lib/api/bindings", () => ({
     playerCancelSleepTimer: (...a: any[]) => playerCancelSleepTimer(...(a as [])),
     playerSetSubtitleTrack: vi.fn(async () => ({})),
     playerSwitchAudioTrack: vi.fn(async () => ({})),
+    // The player loads the streaming-quality picker on mount; without these the
+    // mock throws and every test in the file fails before it starts.
+    playerGetStreamingQualities: vi.fn(async () => []),
+    playerGetVideoSettings: vi.fn(async () => ({ streamingQuality: "original" })),
     storageGetSeriesAudioPreference: vi.fn(async () => null),
     storageSaveSeriesAudioPreference: vi.fn(async () => ({})),
   },