fix(player): make the video seek bar work by touch (DR-099)

On Android, dragging or tapping the progress bar moved the thumb but
playback stayed where it was. Two separate defects, both touch-only,
which is why the mouse-driven scrub tests never caught either.

1. Gesture hijack. DR-098 taught handleTouchStart to ignore touches that
   land on a control, but handleTouchMove kept running. It measures
   against touchStartX/Y, which that early return leaves at the PREVIOUS
   gesture's values, so a seek-bar drag produced a huge bogus vertical
   delta: read as a brightness swipe, it dimmed the screen to the 0.3
   floor and fired a spurious play/pause "correction" mid-drag. A gesture
   is now latched at touchstart (playerGestureActive) and touchmove
   ignores anything unlatched — re-checking the move target cannot
   recover a start point that was never recorded.

2. Commit signal. The seek was committed only from `change`, which
   Android's WebView does not reliably fire for a touch interaction on a
   range input, so the thumb moved to the tapped position and no seek
   ever ran. touchend/mouseup now commit too; `input` arms a one-shot
   latch so whichever release signal arrives first commits and the other
   is a no-op. seekRelative shares the same commitSeek entry point
   instead of fabricating a synthetic change event.

Tests drive the slider with real touch events (UT-089, UT-090) and fail
against the pre-fix component.

src/lib/components/player/VideoPlayer.svelte

@@ -1,4 +1,4 @@
-<!-- TRACES: UR-003, UR-005, UR-020, UR-021, UR-026, UR-040, UR-061 | DR-010, DR-023, DR-024, DR-051, DR-052, DR-092 -->
+<!-- TRACES: UR-003, UR-005, UR-020, UR-021, UR-026, UR-040, UR-061 | DR-010, DR-023, DR-024, DR-051, DR-052, DR-092, DR-098, DR-099 -->
 <script lang="ts">
   import { onMount, onDestroy, untrack } from "svelte";
   import { goto } from "$app/navigation";
@@ -124,6 +124,14 @@
   // so back-to-back double taps chain instead of stacking on a stale position.
   let pendingSeekTarget: number | null = null;
   let swipeGestureActive = $state(false);
+  // Whether the in-flight touch belongs to the player surface (and so may be
+  // read as a tap/swipe gesture) rather than to a control. Set on touchstart,
+  // cleared on touchend — see handleTouchMove for why a per-gesture flag and not
+  // just a per-event target check.
+  let playerGestureActive = false;
+  // Raised when the user changes the seek bar's value, cleared by whichever
+  // release signal commits the seek. See handleSeekBarRelease.
+  let seekCommitArmed = false;
 
   // Backend info from Rust (Rust decides which backend to use based on platform)
   let useHtml5Element = $state(true); // Default to HTML5, Rust will override if using native backend
@@ -1162,14 +1170,33 @@
     const targetTime = parseFloat(input.value);
     // Update the displayed time immediately for smooth visual feedback
     currentTime = targetTime;
+    // The user has moved the value; the next release must commit it.
+    seekCommitArmed = true;
   }
 
-  async function handleSeekBarChange(e: Event) {
-    const input = e.target as HTMLInputElement;
+  /**
+   * Seek-bar released — commit the value the user landed on, at most once.
+   *
+   * Wired to `touchend`/`mouseup` AND `change`, because `change` alone is not
+   * dependable: Android's WebView does not reliably fire it for a touch
+   * interaction on a range input, so the thumb moved to the tapped position but
+   * the seek never ran ("the bar moves, playback doesn't"). Engines that DO fire
+   * `change` deliver both signals, hence the arm/disarm — whichever arrives
+   * first commits and the other is a no-op.
+   */
+  function handleSeekBarRelease(e: Event) {
+    isDraggingSeekBar = false;
+    if (!seekCommitArmed) return;
+    seekCommitArmed = false;
+    const input = (e.currentTarget ?? e.target) as HTMLInputElement;
+    void commitSeek(parseFloat(input.value));
+  }
+
+  async function commitSeek(rawTarget: number) {
     // Clamp strictly inside the media: the range input's max IS the duration, so
     // dragging fully right would otherwise request a segment past the media end,
     // which the server never produces (see END_SEEK_MARGIN_SECONDS).
-    const targetTime = clampSeekTarget(parseFloat(input.value), duration);
+    const targetTime = clampSeekTarget(rawTarget, duration);
 
     // Set isSeeking immediately to prevent timeupdate from interfering
     isSeeking = true;
@@ -1406,16 +1433,9 @@
       to: newTime.toFixed(2),
     });
 
-    // Call the unified handleSeekBarChange logic with the new time
-    // Create a synthetic event to reuse the existing logic
-    const syntheticEvent = {
-      target: {
-        value: newTime.toString()
-      }
-    } as unknown as Event;
-
+    // Same commit path as the seek bar — one place decides how a seek is issued.
     try {
-      await handleSeekBarChange(syntheticEvent);
+      await commitSeek(newTime);
     } finally {
       // The player is authoritative again from here on.
       if (pendingSeekTarget === newTime) pendingSeekTarget = null;
@@ -1473,7 +1493,16 @@
     // container and touch events bubble, so without this a tap on the bottom
     // play button would toggle here AND again via the button's own click — the
     // two cancelling out and leaving the control apparently dead (DR-098).
-    if (isControlSurfaceTouch(ancestorChain(e.target))) return;
+    if (isControlSurfaceTouch(ancestorChain(e.target))) {
+      // The move handler must stay out of it too. It reads touchStartX/Y, which
+      // this early return leaves at the PREVIOUS gesture's values, so a seek-bar
+      // drag came out as a huge vertical delta: it was mis-read as a brightness
+      // swipe, which dimmed the screen and fired a spurious play/pause
+      // "correction" mid-drag (DR-098).
+      playerGestureActive = false;
+      return;
+    }
+    playerGestureActive = true;
 
     const touch = e.touches[0];
     touchStartX = touch.clientX;
@@ -1505,6 +1534,10 @@
   }
 
   function handleTouchMove(e: TouchEvent) {
+    // Only a gesture that began on the bare video surface is ours. Re-checking
+    // the target here would not be enough: the touch that started on a control
+    // never recorded a start point, so any delta computed here is meaningless.
+    if (!playerGestureActive) return;
     if (!e.touches[0]) return;
 
     const touch = e.touches[0];
@@ -1537,6 +1570,7 @@
   }
 
   function handleTouchEnd(e: TouchEvent) {
+    playerGestureActive = false;
     swipeGestureActive = false;
     swipeType = null;
   }
@@ -1900,11 +1934,11 @@
           max={duration || 100}
           value={currentTime}
           oninput={handleSeekBarInput}
-          onchange={handleSeekBarChange}
+          onchange={handleSeekBarRelease}
           onmousedown={() => isDraggingSeekBar = true}
-          onmouseup={() => isDraggingSeekBar = false}
+          onmouseup={handleSeekBarRelease}
           ontouchstart={() => isDraggingSeekBar = true}
-          ontouchend={() => isDraggingSeekBar = false}
+          ontouchend={handleSeekBarRelease}
           class="flex-1 h-1 bg-white/30 rounded-full appearance-none cursor-pointer
                  [&::-webkit-slider-thumb]:appearance-none [&::-webkit-slider-thumb]:w-3 [&::-webkit-slider-thumb]:h-3
                  [&::-webkit-slider-thumb]:bg-white [&::-webkit-slider-thumb]:rounded-full"