fix(player): keep double-tap seek working over the play overlay (DR-098)

The control-surface guard added in the previous commit killed
double-tap-to-seek. The first tap pauses, which renders the full-screen
<button> play overlay over the video, so the SECOND tap lands on a
button — and the guard discarded it as "a tap on a control".

Mark that overlay `data-player-surface`: visually it IS the video, so it
must keep taking tap gestures despite being a <button>. The marker wins
over the interactive-tag check in isControlSurfaceTouch.

Adds VideoPlayer.tapSurface.test.ts, which renders the REAL component
and dispatches real touch/click events at whatever element is genuinely
on top. This is the gap that let four bugs ship in a row: the pure-unit
tests over registerTap/isControlSurfaceTouch/isSynthesizedTouchClick all
passed throughout, because each helper behaved exactly as specified —
every bug was in the composition, i.e. which element actually receives a
tap after Svelte re-renders. Modelling that DOM by hand in a test would
just re-encode the same wrong assumption, so these render it instead.

The new double-tap test was verified to fail with the fix reverted and
pass with it applied, in both directions.

src/lib/components/player/tapGestures.ts

@@ -45,12 +45,19 @@ export const TOUCH_CLICK_SUPPRESS_MS = 700;
  * testable without a DOM.
  */
 export function isControlSurfaceTouch(
-  ancestors: Array<{ tag: string; isPlayerControls?: boolean }>
+  ancestors: Array<{ tag: string; isPlayerControls?: boolean; isPlayerSurface?: boolean }>
 ): boolean {
   const INTERACTIVE = new Set(["button", "a", "input", "select", "textarea", "label"]);
-  return ancestors.some(
-    (node) => node.isPlayerControls === true || INTERACTIVE.has(node.tag.toLowerCase())
-  );
+  for (const node of ancestors) {
+    // `data-player-surface` wins over the tag check: the full-screen play overlay
+    // is a <button> but is visually the video itself, and must keep taking tap
+    // gestures — otherwise the second tap of a double tap (which lands on it,
+    // because the first tap paused and raised it) is discarded and seeking dies.
+    if (node.isPlayerSurface === true) return false;
+    if (node.isPlayerControls === true) return true;
+    if (INTERACTIVE.has(node.tag.toLowerCase())) return true;
+  }
+  return false;
 }
 
 /**