fix(player): decide transport in Rust for webview media (DR-097)

Video on Android/Linux renders in a webview <video> element, and the
frontend facade short-circuited play/pause/toggle straight into the
adapter whenever one was registered. Html5PlayerAdapter.toggle() then
decided play-vs-pause by reading el.paused off the DOM, so the Rust
controller never saw the intent and could not serialise competing ones.

el.paused flips transiently while an element buffers or settles a seek.
Two intents ~150ms apart therefore read *different* values and performed
*opposing* actions — one playing, one pausing — which self-sustained a
play/pause loop that needed no further input. On device this showed up
as a fully healthy element (readyState=4, networkState=1, not seeking,
not buffering, not ended) pausing itself roughly once a second, so
unpausing or skipping ahead bounced straight back to paused.

The root cause was that Rust held NO state for webview-rendered media:
report_html5_state only re-emitted its argument, despite the comment
above it claiming the controller was the single source of truth. It had
nothing to decide a toggle from.

Now report_html5_state tracks the reported state, and play/pause/toggle
consult it and drive the element by emitting a ControlCommand — the same
"backend decides, adapter executes the primitive" split player_seek_video
already uses. A stopped/idle report clears the tracking so MPV/ExoPlayer
regain authority for music playback.

Tests cover the loop signature directly (repeated toggles must alternate,
never repeat or oppose) plus a guard that one intent yields exactly one
ControlCommand — which matters on Windows, where the backend is itself
webview-based and could otherwise be driven twice.

src/lib/player/index.ts

@@ -12,7 +12,7 @@
  * derived + merged (remote-session-aware) stores so UI can import state and
  * actions from one place, in both local and remote modes.
  *
- * TRACES: UR-005 | DR-001, DR-009
+ * TRACES: UR-005 | DR-001, DR-009, DR-097 | UT-089
  */
 
 import { get } from "svelte/store";
@@ -83,18 +83,29 @@ function requireHandle(): string {
 // Transport controls (no repository handle required)
 // ---------------------------------------------------------------------------
 
+// Transport intents ALWAYS go to the backend, in both native and HTML5 modes.
+//
+// These used to short-circuit into the active video adapter, which made the
+// webview the decider: `adapter.toggle()` read `el.paused` off the DOM and
+// flipped the element, so Rust never saw the intent. `el.paused` flips
+// transiently while an element buffers or settles a seek, so two intents
+// ~150ms apart could read different values and take opposing actions — a
+// self-sustaining play/pause loop.
+//
+// Now Rust decides from PlayerController state and drives the element back
+// through a `ControlCommand` event (handled in playerEvents.ts), the same
+// "backend decides, adapter executes the primitive" split used by
+// player_seek_video. Do NOT reintroduce an adapter short-circuit here.
+
 async function play() {
-  if (activeAdapter) return void (await activeAdapter.play());
   await commands.playerPlay();
 }
 
 async function pause() {
-  if (activeAdapter) return void (await activeAdapter.pause());
   await commands.playerPause();
 }
 
 async function toggle() {
-  if (activeAdapter) return void (await activeAdapter.toggle());
   await commands.playerToggle();
 }