Fast Piggie: add 15ms pre-flash gap before image reveal

[Copilot]

At high speeds, Fast Piggie wedge visuals could still be present when the next image flash began, making selection cues overlap with the stimulus. This change introduces a small pre-flash separation so wedges disappear before images are shown.

• Timing behavior

  • Added a 15ms lead-in between board clear (wedges-only state) and image flash start.
  • _runRound() now clears the board first, then schedules image draw after the lead-in.

• Timer lifecycle safety

  • Split timer responsibilities into:
    • pre-flash delay timer
    • image display-duration timer
  • Updated stop() and reset() to clear both timers, preventing in-flight callbacks during transitions.

• Focused coverage

  • Added a timing-sequence test to verify:
    • first timeout is scheduled for 15ms
    • post-delay timeout is scheduled for the mocked round display duration

clearImages(_ctx, width, height, wedgeCount);

_imageFlashTimer = setTimeout(() => {
  drawBoard(/* show images */);

  _roundTimer = setTimeout(() => {
    clearImages(_ctx, width, height, wedgeCount);
    _clickEnabled = true;
  }, displayDurationMs);
}, ROUND_IMAGE_FLASH_DELAY_MS);