feat(player): native video on Linux, and one contract for every player (v0.11.0)

mpv now decodes video on Linux, drawn into a framebuffer we own and blitted
into the default vbox's draw handler. Tauri's widget tree is untouched, so an
upgrade that assumes its own layout cannot invalidate this. Direct play means
the original file, hardware decoding, and no server transcode at all — where
previously every desktop video was re-encoded to h264 for the browser engine,
whatever the file actually was. Off by default: JELLYTAU_NATIVE_VIDEO=1.

That settles finding 2 of playback-backend-unification.md — "native video
cannot be composited with a Tauri webview" — by demonstration rather than
argument, on X11 and Wayland both.

Turning it on exposed nine defects, none of them mpv's. Each was the same
mistake in a different place: a capability written down as a compile-time fact
about the platform, or a state asserted instead of confirmed.

  DR-238/246  a seek routed by the stream's container rather than by what the
              engine could do with it - correct only while one player handled
              those streams, silent the moment another did
  DR-239      a property handled but never observed, so the play/pause button
              waited for an event that could not arrive
  DR-240      fullscreen expanding the document while the window stayed put
  DR-241      a seek issued before the engine had a file, failed, and discarded
              - which is why resume began at zero
  DR-247      a Linux-only gate outliving the caller that made it Linux-only,
              breaking the Android build outright
  DR-250      a stop aimed at whichever renderer bookkeeping believed was in
              charge, missing the one actually making sound
  DR-251      a duration of zero believed, leaving the seek bar no scale
  DR-252      a junk float converted to a Duration, panicking the backend the
              instant a length-less stream appeared

So the MediaPlayer contract (DR-242 … DR-247): `open` carries a start position,
so no caller sequences load-then-seek and none can race an engine's load;
`seek` states a destination and leaves in-place-versus-re-open to the engine;
`snapshot` is one coherent read; and `Phase::Opening` names the window where
intent used to be lost. One conformance suite runs against every engine —
FakePlayer and mpv under cargo test, ExoPlayer instrumented on a device — so an
engine is either correct or visibly failing.

Two of the nine were introduced during this work and caught on hardware, not by
any suite: an over-broad capability that grouped ExoPlayer with mpv, and the
Duration panic. The suites test engines that behave. That is recorded in
docs/native-player-verification.md, which asks for the exact action sequences
that found them.

Verified: all automated gates, conformance (mpv 9/9, legacy 8/9 by design,
ExoPlayer 7/7 on device), and manual desktop and Android passes on real
hardware.

Known open and deliberately shipped: resume reads local progress and never the
server's; the background-audio handoff still declares a state swap it does not
confirm (the symptom is now impossible, the race is not); and `bun run
android:dev` builds an APK carrying the release application id, whose failure
message advises an uninstall that would destroy app data. Fix that last one
before anyone else builds for Android.

Squashed from worktree-linux-native-video, which keeps the per-defect history.

src-tauri/src/player/native_video.rs

@@ -0,0 +1,78 @@
+//! Whether this process renders video natively, answered once.
+//!
+//! Three things need this and must agree: the mpv backend (which has to be
+//! configured for video *at construction*, before anything plays), the video
+//! surface (which has nothing to draw otherwise), and `get_player_status`
+//! (which tells the frontend whether to use a webview `<video>` element).
+//!
+//! It is a function rather than three `env::var` checks for the reason this
+//! codebase keeps rediscovering: a capability answered in several places is a
+//! capability whose answers drift. Four separate bugs this cycle came from
+//! exactly that shape — a webview's decode limits applied to ExoPlayer, a
+//! transcode target contradicting a direct-play claim, a codec list hardcoded in
+//! a URL builder. One source, read by everyone.
+//!
+//! TRACES: UR-080 | DR-231, DR-235
+
+/// The opt-in for native desktop video.
+///
+/// Off by default while the render path is unproven — the webview path still
+/// works and is what ships. This becomes the *default* (and then the only path)
+/// when DR-235 lands; the variable is how it is exercised until then.
+const ENV_FLAG: &str = "JELLYTAU_NATIVE_VIDEO";
+
+/// Whether mpv should decode and draw video in this process.
+///
+/// Read fresh rather than cached: it is consulted a handful of times at startup,
+/// and a `OnceLock` here would only make it harder to test.
+///
+/// TRACES: UR-080 | DR-231, DR-235
+pub fn enabled() -> bool {
+    // Only where a native renderer exists. On Android ExoPlayer already does
+    // this and `use_html5_element` is false for entirely separate reasons.
+    if !cfg!(all(target_os = "linux", not(target_os = "android"))) {
+        return false;
+    }
+    matches!(
+        std::env::var(ENV_FLAG).as_deref(),
+        Ok("1") | Ok("true") | Ok("yes")
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Absent, empty, or anything unrecognised means off. A half-set variable
+    /// must not half-enable a renderer — the failure mode would be mpv
+    /// configured for video with nothing drawing it, i.e. audio playing over a
+    /// black rectangle.
+    ///
+    /// TRACES: UR-080 | DR-231 | UT-216
+    #[test]
+    fn test_only_explicit_truthy_values_enable_it() {
+        let restore = std::env::var(ENV_FLAG).ok();
+
+        for value in ["", "0", "no", "false", "maybe", "2"] {
+            std::env::set_var(ENV_FLAG, value);
+            assert!(!enabled(), "{value:?} must not enable native video");
+        }
+
+        for value in ["1", "true", "yes"] {
+            std::env::set_var(ENV_FLAG, value);
+            assert_eq!(
+                enabled(),
+                cfg!(all(target_os = "linux", not(target_os = "android"))),
+                "{value:?} enables it exactly where a native renderer exists"
+            );
+        }
+
+        std::env::remove_var(ENV_FLAG);
+        assert!(!enabled(), "absent means off");
+
+        match restore {
+            Some(v) => std::env::set_var(ENV_FLAG, v),
+            None => std::env::remove_var(ENV_FLAG),
+        }
+    }
+}