Fast Piggie: fix outlier z-order and min display time tracking

[Copilot]

Two UX bugs in Fast Piggie: the target image could be obscured by distractors at high image counts, and the game card "Min Display Time" stat was showing answer response time (~87ms) instead of the actual minimum display duration (~10ms).

Outlier image z-order (index.js → drawBoard)

Canvas 2D z-order is draw order. Previously all images were drawn wedge-by-wedge, so distractors rendered after the outlier would cover it. Fix defers the outlier's drawImage call until after the loop completes, ensuring it always renders on top.

// Save outlier draw params — rendered last to appear on top.
outlierDraw = { entry, imgCx, imgCy, drawH, drawW };
// ...after loop:
if (outlierDraw) ctx.drawImage(/* outlierDraw params */);

Min display time tracking (game.js + index.js)

lowestDisplayTime was being populated from topSpeedMs (time-to-click after images hide), not the actual round display duration. Introduces lowestRoundDisplayMs to track the minimum displayDurationMs across all rounds, passed from _currentRound into addScore/addMiss.

• addScore(guineaPigs, answerSpeedMs, displayDurationMs) — new third param
• addMiss(guineaPigs, displayDurationMs) — new second param
• getBestStats() and stopGame() now expose lowestRoundDisplayMs
• stop() uses bestStats.lowestRoundDisplayMs → lowestDisplayTime when saving score