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.

docs/architecture/03-data-flow.md

@@ -132,6 +132,55 @@ sequenceDiagram
     Note over Store: UI updates reactively
 ```
 
+## Video Stream Selection Flow
+
+**TRACES: UR-070, UR-079 | DR-224, DR-226, DR-227**
+
+Before a video plays, Rust decides *what stream* — direct play, remux or
+transcode, over which transport — and hands the player one self-describing
+`StreamSelection`. The page no longer inspects the URL to work any of this out.
+
+```mermaid
+sequenceDiagram
+    participant Page as player/[id]/+page.svelte
+    participant Repo as HybridRepository
+    participant Online as OnlineRepository
+    participant Server as Jellyfin
+    participant VP as VideoPlayer.svelte
+
+    Page->>Repo: playerLocalMediaPath(id)
+    alt a completed download exists
+        Page->>Repo: mediaLocalSelection(path)
+        Note over Page: LocalFile / DirectPlay, no ladder —<br/>nothing about a file on disk re-negotiates
+    else stream from the server
+        Page->>Repo: getStreamSelection(id, mediaSourceId)
+        Repo->>Online: get_stream_selection()
+        Online->>Online: effective_streaming_quality()
+        Note over Online: per-playback override, else device default
+        Online->>Server: POST /Items/{id}/PlaybackInfo<br/>(device profile + ceiling)
+        Server-->>Online: MediaSource {supportsDirectPlay,<br/>supportsDirectStream, transcodingUrl, bitrate}
+        Online->>Online: decide_playback_kind()
+        alt Transcode
+            Online->>Online: adopt/stop prior play session,<br/>build HLS URL
+            Note over Online: Transport::Hls
+        else DirectPlay / DirectStream
+            Online->>Online: /Videos/{id}/stream?static=true
+            Note over Online: Transport::Progressive,<br/>rendition = None (it IS the source)
+        end
+        Online->>Online: quality_options_for_source(bitrate)
+        Online-->>Page: StreamSelection
+    end
+    Page->>VP: selection
+    VP->>VP: videoLoaderFor(selection, caps)
+    Note over VP: hls.js / native HLS / direct —<br/>from the tag, never from the URL
+```
+
+The selection travels with the stream from then on. A reload — a quality change,
+an audio-track switch, a transcoded seek — returns a *new* selection through the
+same tagged `strategy` response, so transport and URL can never disagree; and the
+queue item carries the transport so `player_seek_video` picks its seek strategy
+from the backend's decision rather than from the URL string.
+
 ## Playback Mode Transfer Flow
 
 ```mermaid