feat(playback): let Rust decide what stream to play, and say so

Playing a video meant asking the server to re-encode it, always. That
decision was made nowhere and written down nowhere, so whoever needed it
re-derived it downstream — the player worked out whether it had been handed
a playlist by looking for ".m3u8" in the URL, in two places. A viewer paid
for a transcode of a file their device could have played untouched, and the
app could not tell them which it was.

One negotiation now produces one self-describing StreamSelection — direct
play, remux or transcode; over a playlist, a plain HTTP file, or a local one
— and every renderer consumes that same answer.

Measured against the development server (Jellyfin 10.11.5), 400 items
sampled for codec mix and 40 put through a real PlaybackInfo negotiation
per profile:

  Linux / WebKitGTK (h264 only, 2ch)          3/40 —  7% direct play
  Android / ExoPlayer (hevc, ac3/eac3, 6ch)  34/40 — 85% direct play

The library is ~80% hevc, which is why the two diverge so hard. The payoff
is overwhelmingly Android, where 85% of plays were starting a transcode
nobody needed. Linux stays near 7% until libmpv decodes the picture — the
h264-only profile is a WebKitGTK constraint, not a JellyTau choice.

DR-219  StreamSelection: url + tagged Transport (hls/progressive/localFile)
        + PlaybackKind (directPlay/directStream/transcode) + the negotiated
        rendition + this source's ladder + a needs_transcoding flag derived
        in Rust so the rule is answered once. Both enums are serde-tagged
        so the frontend matches a discriminant, not a substring. The paths
        that never negotiate get the same shape from Rust rather than
        assembling one — media_local_selection for a downloaded file,
        LiveStreamInfo.transport for a live channel — so there is no second
        place where a transport is decided.

DR-220  The ceiling becomes two levels: a durable device default (Settings,
        persisted) and a per-playback override the in-player picker sets.
        The picker had called itself a "this film, this connection" control
        since it was written but wrote the process-wide default, so dropping
        one awkward film to 2 Mbps silently capped every video played
        afterwards for the rest of the process, with Settings still showing
        the old value. The override is cleared whenever playback moves to a
        new item, which stops it surviving into an autoplayed next episode.
        effective_streaming_quality() is the single resolution point.

DR-221  The quality picker is filled from what this media source can offer.
        Rust marks a rung exceeds_source when its ceiling is at or above the
        source's own bitrate — such a rung is another way to spell Original
        — and the frontend does not draw those. Original is never marked; a
        source whose bitrate the server does not report marks nothing, which
        keeps every rung offered.

DR-222  Direct play and direct stream are negotiated, with two client-side
        overrides on top because the server's answer is right about the file
        and wrong about what this app will do with it: undecodable audio
        (Jellyfin 10.11.5 honours a DirectPlayProfile's container and video
        codec but ignores its audio codec, so it offers direct play for an
        E-AC-3 track the webview renders in silence) and a viewer-pinned
        audio track the file does not default to. A direct stream is a remux
        and is deliberately not counted as transcoding.

DR-223  Dropped on measurement, not deferred. A master playlist from this
        server carries exactly one EXT-X-STREAM-INF: Jellyfin builds it from
        the single rendition the request asked for rather than publishing a
        ladder. So there is no adaptation for hls.js to be preserving and
        none mpv would lose — the claim that there was, in
        playback-backend-unification.md, does not hold. Recorded rather than
        deleted because it is a measurement: a server that does publish a
        ladder would change the answer.

DR-224  Every backend consumes the same selection. The queue item carries
        the transport, so player_seek_video picks its seek strategy from the
        backend's decision instead of the last stream_url.contains(".m3u8")
        in the codebase. Items queued by a path that never negotiated carry
        None and fall back to needs_transcoding, which is exact rather than
        a guess because every transcode this app requests is HLS (DR-140).

The frontend loader decision moves to streamTransport.ts so it can be
tested: the two cases that pin it are the ones that failed against the old
implementation — a progressive stream whose URL contains ".m3u8" must not
get an HLS loader, and an HLS stream whose URL contains none must.

Also verified the URL the direct-play branch builds actually serves playable
bytes: 206, video/mp4, valid ISO-BMFF, and a mid-file range works, so
seeking a direct play works.

The spec is folded into docs/architecture/{01,02,03} and deleted, per the
rule that docs/specs holds only work that has not shipped. DR-121 leaves
read-through-media-cache.md with a pointer; that spec keeps its capture half.

Not verified: real playback on a device. Direct play changes what actually
gets played, and neither fixtures nor curl prove the WebKitGTK and ExoPlayer
paths render it.

src/lib/player/adapters/html5Adapter.test.ts

@@ -10,6 +10,23 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 import { Html5PlayerAdapter, type Html5ElementBridge } from "./html5Adapter";
 import type { AdapterHost } from "./types";
 
+/**
+ * A `StreamSelection` for tests that only care about the URL. Transcoded HLS is
+ * what these paths exercised before the contract carried a transport.
+ */
+function testSelection(url: string, transport: "hls" | "progressive" | "localFile" = "hls") {
+  return {
+    url,
+    transport: { type: transport },
+    playbackKind: { type: transport === "hls" ? "transcode" : "directPlay" },
+    rendition: null,
+    available: [],
+    mediaSourceId: null,
+    playSessionId: null,
+    needsTranscoding: transport === "hls",
+  } as import("$lib/api/bindings").StreamSelection;
+}
+
 /** A minimal fake <video> element that records mutations and fires events. */
 function makeFakeVideo() {
   const listeners: Record<string, Array<() => void>> = {};
@@ -54,7 +71,7 @@ function makeBridge(overrides: Partial<Html5ElementBridge> = {}): Html5ElementBr
     setSeekOffset: vi.fn((o: number) => {
       offset = o;
     }),
-    setStreamUrl: vi.fn(),
+    setStreamSelection: vi.fn(),
     destroyHls: vi.fn(),
     getMediaSourceId: () => "msid-1",
     ...overrides,
@@ -184,7 +201,7 @@ describe("Html5PlayerAdapter", () => {
 
   it("reloadSource() runs the invariant teardown->swap->resume sequence", async () => {
     video.paused = false; // was playing → should resume
-    const p = adapter.reloadSource("http://new/master.m3u8", 120);
+    const p = adapter.reloadSource(testSelection("http://new/master.m3u8"), 120);
 
     // Teardown happened synchronously before the awaited canplay wait.
     expect(video.pause).toHaveBeenCalled();
@@ -194,7 +211,9 @@ describe("Html5PlayerAdapter", () => {
 
     // Allow the internal 100ms settle delay, then fire canplay to resume.
     await new Promise((r) => setTimeout(r, 110));
-    expect(bridge.setStreamUrl).toHaveBeenCalledWith("http://new/master.m3u8");
+    expect(bridge.setStreamSelection).toHaveBeenCalledWith(
+      expect.objectContaining({ url: "http://new/master.m3u8", transport: { type: "hls" } }),
+    );
     video._fire("canplay");
     video._fire("seeked");
     await p;
@@ -217,7 +236,7 @@ describe("Html5PlayerAdapter", () => {
    */
   it("reloadSource() seeks to the position and clears the transcode offset", async () => {
     video.paused = false;
-    const p = adapter.reloadSource("http://new/master.m3u8", 1200);
+    const p = adapter.reloadSource(testSelection("http://new/master.m3u8"), 1200);
 
     await new Promise((r) => setTimeout(r, 110));
     expect(bridge.setSeekOffset).toHaveBeenCalledWith(0);
@@ -238,7 +257,7 @@ describe("Html5PlayerAdapter", () => {
   /** A reload to the very start has nothing to seek to; it must not stall. */
   it("reloadSource() at position 0 does not wait for a seek", async () => {
     video.paused = false;
-    const p = adapter.reloadSource("http://new/master.m3u8", 0);
+    const p = adapter.reloadSource(testSelection("http://new/master.m3u8"), 0);
     await new Promise((r) => setTimeout(r, 110));
     video._fire("canplay");
     await p; // resolves without any "seeked" event
@@ -258,7 +277,7 @@ describe("Html5PlayerAdapter", () => {
     vi.useFakeTimers();
     try {
       video.paused = false;
-      const p = adapter.reloadSource("http://new/master.m3u8", 120);
+      const p = adapter.reloadSource(testSelection("http://new/master.m3u8"), 120);
       const assertion = expect(p).rejects.toThrow(/canplay/i);
       await vi.advanceTimersByTimeAsync(11_000); // past the 10s readiness budget
       await assertion;
@@ -270,7 +289,7 @@ describe("Html5PlayerAdapter", () => {
 
   it("reloadSource() does not resume when it was paused", async () => {
     video.paused = true;
-    const p = adapter.reloadSource("http://new/master.m3u8", 30);
+    const p = adapter.reloadSource(testSelection("http://new/master.m3u8"), 30);
     await new Promise((r) => setTimeout(r, 110));
     video._fire("canplay");
     video._fire("seeked");