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/services/playerEvents.ts

@@ -5,7 +5,7 @@
  * frontend stores accordingly. This enables push-based updates instead
  * of polling.
  *
- * TRACES: UR-005, UR-019, UR-023, UR-026 | DR-001, DR-028, DR-047
+ * TRACES: UR-005, UR-019, UR-023, UR-026 | DR-001, DR-028, DR-047, DR-097
  */
 
 import { type UnlistenFn } from "@tauri-apps/api/event";
@@ -306,9 +306,15 @@ function handleSleepTimerChanged(mode: SleepTimerMode, remainingSeconds: number)
 
 /**
  * Route a backend-originated control command to the active player adapter, so a
- * backend intent (lockscreen/remote/sleep) can drive the webview <video> element
- * that Rust cannot reach directly. No-op when no video adapter is active (audio
- * playback is already fully backend-driven).
+ * backend intent can drive the webview <video>/<audio> element that Rust cannot
+ * reach directly. No-op when no adapter is active (native playback is already
+ * fully backend-driven).
+ *
+ * This is the EXECUTION half of transport authority: for webview-rendered media
+ * the Rust controller decides play-vs-pause from the state the element reported
+ * and emits it here as a ControlCommand. UI intents go *to* the backend (see the
+ * facade in $lib/player) and come back through this path — never short-circuited
+ * in the webview, which is what caused the DR-097 pause loop.
  */
 function handleControlCommand(action: string, position: number | null): void {
   const adapter = playerController.getActiveAdapter();