feat(player): seek strategy follows what the engine says it can do

DR-246. The strategy used to turn on `is_hls` and `use_html5`, decided in a
command handler on behalf of engines it does not own. That is how "who
renders" came to mean "how do I seek", and why a transcoded seek silently did
nothing the moment native video changed the renderer (DR-238).

Engines now declare `Capabilities::seeks_transcoded_in_place` — true for
hls.js, which seeks within the VOD playlist it was handed and lets the server
catch up; false for mpv, whose HLS demuxer cannot make the server transcode
from a new offset. The command asks whichever engine is rendering. Adding an
engine no longer means editing a shared truth table.

The item's transport is not read at the seek site any more; the compiler
flagged it unused, which is the URL-shape input finally disappearing.

A deviation from the spec, recorded deliberately: it called for the engine to
own the decision outright. It cannot. Re-negotiating a stream needs the
repository, which sits above the engine, so the engine states the ability and
the caller acts on it. That still removes the defect — nobody guesses on
another component's behalf — without pretending an engine can reach upward.

Also fixes a latent race in the conformance suite, found by running it: the
seek case asserted immediately, which passes on an engine that records the
target when it accepts a seek and races on one that waits for the decoder to
move. `Harness::await_seek` polls instead, the way the Android suite already
did. It failed with machine load rather than with the code, which is the kind
of test that teaches people to re-run until green.

  MpvPlayer     9/9
  LegacyPlayer  8/9 - still only the mute/rate gap in the old trait

789 tests, clippy -D warnings clean with and without the feature.

src-tauri/src/player/seek.rs

@@ -25,12 +25,14 @@ pub enum VideoSeekStrategy {
 ///
 /// # Arguments
 /// * `is_local` - Whether the file is a local download
-/// * `is_hls` - Whether the stream URL contains ".m3u8" (HLS stream)
+/// * `seeks_transcoded_in_place` - Whether the engine rendering this stream
+///   can seek a server-side transcode without re-opening it. Declared by the
+///   engine via `Capabilities`, never inferred from the URL or the renderer.
 /// * `needs_transcoding` - Whether the content needs transcoding
 /// * `use_html5` - Whether frontend is using HTML5 video element
 pub fn determine_video_seek_strategy(
     is_local: bool,
-    is_hls: bool,
+    seeks_transcoded_in_place: bool,
     needs_transcoding: bool,
     use_html5: bool,
 ) -> VideoSeekStrategy {
@@ -53,14 +55,15 @@ pub fn determine_video_seek_strategy(
     // native video on routed every transcoded seek into a backend seek that
     // silently does nothing, and presents as "resume does not work".
     if needs_transcoding {
-        return if use_html5 {
-            if is_hls {
-                VideoSeekStrategy::Html5NativeSeek
-            } else {
-                VideoSeekStrategy::Html5ReloadStream
-            }
-        } else {
-            VideoSeekStrategy::BackendReloadStream
+        // Whether a transcode can be seeked in place is a property of the
+        // engine, and the engine states it. This used to be inferred from
+        // `is_hls`, which held only while hls.js was the sole HLS renderer —
+        // and stopped holding the moment mpv became one (DR-238).
+        return match (seeks_transcoded_in_place, use_html5) {
+            (true, true) => VideoSeekStrategy::Html5NativeSeek,
+            (true, false) => VideoSeekStrategy::BackendNativeSeek,
+            (false, true) => VideoSeekStrategy::Html5ReloadStream,
+            (false, false) => VideoSeekStrategy::BackendReloadStream,
         };
     }
 
@@ -235,20 +238,21 @@ mod tests {
         );
     }
 
-    /// Test video seek strategy for HLS streams
+    /// Non-transcoded streams seek in place regardless of the engine's
+    /// transcode ability, which only applies to transcodes.
     #[test]
-    fn test_seek_strategy_hls_stream() {
-        // HLS with HTML5 - frontend handles seek, don't call backend
+    fn test_seek_strategy_direct_stream() {
+        // HTML5 renders, so the frontend seeks the element
         assert_eq!(
             determine_video_seek_strategy(false, true, false, true),
             VideoSeekStrategy::Html5NativeSeek
         );
-        // HLS with native backend - backend handles seek
+        // The native engine renders, so it seeks
         assert_eq!(
             determine_video_seek_strategy(false, true, false, false),
             VideoSeekStrategy::BackendNativeSeek
         );
-        // HLS even with needs_transcoding flag - still native seek (HLS supports it)
+        // A transcode an engine says it can move: seek in place
         assert_eq!(
             determine_video_seek_strategy(false, true, true, true),
             VideoSeekStrategy::Html5NativeSeek
@@ -265,18 +269,30 @@ mod tests {
     /// every transcoded seek into a native seek that silently does nothing,
     /// which presents as "resume does not work".
     ///
-    /// TRACES: UR-040 | DR-238 | UT-217
+    /// TRACES: UR-040 | DR-238, DR-246 | UT-217
     #[test]
-    fn test_seek_strategy_transcoded_hls_native_backend() {
+    fn test_transcoded_seek_follows_the_engines_declared_ability() {
+        // An engine that cannot move a server-side transcode re-opens it,
+        // whichever side is rendering.
         assert_eq!(
-            determine_video_seek_strategy(false, true, true, false),
+            determine_video_seek_strategy(false, false, true, false),
             VideoSeekStrategy::BackendReloadStream
         );
-        // The HTML5 side of the same case is unchanged: hls.js seeks in-playlist.
+        assert_eq!(
+            determine_video_seek_strategy(false, false, true, true),
+            VideoSeekStrategy::Html5ReloadStream
+        );
+        // hls.js can, and says so, so it seeks in place.
         assert_eq!(
             determine_video_seek_strategy(false, true, true, true),
             VideoSeekStrategy::Html5NativeSeek
         );
+        // The container the stream arrives in no longer decides anything: the
+        // same declared ability gives the same answer on the native side.
+        assert_eq!(
+            determine_video_seek_strategy(false, true, true, false),
+            VideoSeekStrategy::BackendNativeSeek
+        );
     }
 
     /// Test video seek strategy for direct play (non-transcoded) streams