feat(casting): add DEBUG_TOUCH_ZONES overlay for hit-area calibration

utils/casting/debug.ts

@@ -0,0 +1,8 @@
+/**
+ * Developer flag for visualising touch zones in the casting player.
+ *
+ * Flip to `true` to draw outlines over the slider hit area and the control row,
+ * run the app, hand-calibrate `panHitSlop`, then flip back to `false`.
+ * Every use is gated with `__DEV__` so it can never render in a release build.
+ */
+export const DEBUG_TOUCH_ZONES = false;