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/mod.rs

@@ -5,8 +5,17 @@
 pub mod autoplay;
 pub mod backend;
 pub mod background_policy;
+#[cfg(any(test, feature = "conformance"))]
+pub mod conformance;
 pub mod events;
+#[cfg(any(test, feature = "conformance"))]
+pub mod fake_player;
+#[cfg(test)]
+mod fake_player_conformance;
 pub mod media;
+pub mod media_player;
+#[cfg(target_os = "linux")]
+pub mod mpv_player;
 pub mod queue;
 pub mod seek;
 pub mod session;