Otter Stop: geometric speed progression + consistent image sizing

app/games/otter-stop/game.js

@@ -49,8 +49,12 @@ export function setGoKeys(keys) {
 /** Display interval at level 0 (milliseconds). */
 const BASE_INTERVAL_MS = 1500;
 
-/** How much the interval shrinks per level (milliseconds). */
-const INTERVAL_STEP_MS = 50;
+/**
+ * Geometric decay factor applied to the display interval per level.
+ * Each level multiplies the current interval by this rate, producing large
+ * speed jumps at low levels that taper off as the game becomes faster.
+ */
+const INTERVAL_DECAY_RATE = 0.88;
 
 /** Minimum allowed display interval (milliseconds). */
 const MIN_INTERVAL_MS = 150;
@@ -263,12 +267,14 @@ export function recordResponse(isNoGo, spacePressed) {
 
 /**
  * Return the display interval in milliseconds for the current level.
- * Starts at 700 ms and decreases by 50 ms per level, with a floor of 150 ms.
+ * Uses geometric decay: each level multiplies the base interval by
+ * INTERVAL_DECAY_RATE, producing large speed jumps early and increasingly
+ * smaller increments as the game gets faster. Floored at MIN_INTERVAL_MS.
  *
  * @returns {number} Display interval in milliseconds.
  */
 export function getCurrentIntervalMs() {
-  return Math.max(BASE_INTERVAL_MS - level * INTERVAL_STEP_MS, MIN_INTERVAL_MS);
+  return Math.max(Math.round(BASE_INTERVAL_MS * (INTERVAL_DECAY_RATE ** level)), MIN_INTERVAL_MS);
 }
 
 // ── Getters ───────────────────────────────────────────────────────────────────

app/games/otter-stop/style.css

@@ -69,8 +69,8 @@
 }
 
 .os-stimulus__img {
-  max-width: min(440px, calc(50vmin - 60px));
-  max-height: min(440px, calc(50vmin - 60px));
+  width: 100%;
+  height: 100%;
   object-fit: contain;
   transition: opacity 0.08s ease;
   pointer-events: none;

app/games/otter-stop/tests/game.test.js

@@ -510,16 +510,16 @@ describe('getCurrentIntervalMs()', () => {
     expect(getCurrentIntervalMs()).toBe(1500);
   });
 
-  it('returns 1450 at level 1 (after 3 correct no-go inhibitions)', () => {
+  it('returns 1320 at level 1 (after 3 correct no-go inhibitions)', () => {
     recordResponse(true, false);
     recordResponse(true, false);
     recordResponse(true, false);
-    expect(getCurrentIntervalMs()).toBe(1450);
+    expect(getCurrentIntervalMs()).toBe(1320); // Math.round(1500 * 0.88)
   });
 
   it('returns 150 at a high level (floor clamped)', () => {
-    // 3 no-go correct per level, need level 27 → (1500 - 27*50 = 150)
-    for (let i = 0; i < 81; i += 1) recordResponse(true, false);
+    // 3 no-go correct per level; floor is reached at level 19 (Math.round(1500 * 0.88^19) < 150)
+    for (let i = 0; i < 57; i += 1) recordResponse(true, false);
     expect(getCurrentIntervalMs()).toBe(150);
   });