feat(player): FakePlayer and the conformance suite

DR-243. One set of behaviours every engine must satisfy, written before the
second engine exists so it cannot encode whatever the first happens to do —
which is how three playback implementations drifted apart in the first place.

FakePlayer models the one behaviour that matters most: opening is not
instantaneous. `open` parks in Phase::Opening until complete_open() is called,
so a test can put a seek into that window deliberately. That window is where
DR-241 lived, and it was previously unreachable from any test.

The suite drives readiness through a Harness rather than sleeping — the fake
completes on demand, a real engine waits for its own readiness event. A
timing-dependent suite is worse than none, because it teaches people to
re-run until green.

Nine cases, each naming the defect it prevents:

  opens_at_a_start_position          DR-241 - starts there, never at zero
  seek_while_opening_is_honoured     DR-241 - held, not discarded
  seek_while_opening_overrides_start         later intent wins
  pause_and_play_are_observable      DR-239 - state an engine cannot hide
  close_is_silent_and_idempotent             stopped must mean silent
  close_during_open_never_plays              an open cancelled by close
                                             must not come back to life

`audible()` may return None for engines that cannot answer, which skips the
silence assertions rather than passing them vacuously — an assertion that
cannot fail is worse than an absent one.

Also adds MediaItem::sample: the struct has twenty-odd fields, almost none of
which a given test cares about, and repeating the literal per test is how a
new field ends up added in thirty places.

src-tauri/src/player/media.rs

@@ -198,6 +198,48 @@ impl MediaItem {
     }
 }
 
+impl MediaItem {
+    /// A minimal item for tests.
+    ///
+    /// The struct has twenty-odd fields, almost none of which any given test
+    /// cares about, and repeating the literal per test is how a new field ends
+    /// up added in thirty places. Set what matters on the result.
+    ///
+    /// TRACES: UR-081 | DR-243
+    #[cfg(any(test, feature = "conformance"))]
+    pub fn sample(id: &str, url: &str) -> Self {
+        Self {
+            transport: None,
+            id: id.to_string(),
+            title: id.to_string(),
+            name: None,
+            artist: None,
+            album: None,
+            album_name: None,
+            album_id: None,
+            artist_items: None,
+            artists: None,
+            primary_image_tag: None,
+            image_id: None,
+            item_type: None,
+            playlist_id: None,
+            duration: None,
+            artwork_url: None,
+            media_type: MediaType::Video,
+            source: MediaSource::DirectUrl {
+                url: url.to_string(),
+            },
+            video_codec: None,
+            needs_transcoding: false,
+            video_width: None,
+            video_height: None,
+            subtitles: vec![],
+            series_id: None,
+            server_id: None,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;