Otter Stop: geometric speed progression + consistent image sizing

[Copilot]

Two UX issues with Otter Stop: speed increments were a flat 50 ms/level (too slow to challenge players), and go/no-go stimulus images were displayed ~60 px smaller than their container while feedback images filled it entirely.

Speed progression (game.js)

Replace linear decrement with geometric decay (×0.88 per level):

// Before: flat 50 ms/level
const INTERVAL_STEP_MS = 50;
return Math.max(BASE_INTERVAL_MS - level * INTERVAL_STEP_MS, MIN_INTERVAL_MS);

// After: large early jumps, tapering increments
const INTERVAL_DECAY_RATE = 0.88;
return Math.max(Math.round(BASE_INTERVAL_MS * (INTERVAL_DECAY_RATE ** level)), MIN_INTERVAL_MS);

Level	Before	After	Drop
1	1450 ms	1320 ms	−180 ms
5	1250 ms	793 ms	—
Floor reached	level 27	level 19	—

Image sizing (style.css)

Remove the min(440px, calc(50vmin - 60px)) constraints on .os-stimulus__img and use width: 100%; height: 100%; object-fit: contain — matching how feedback (success/failure) images already fill their container.

Tests (tests/game.test.js)

Updated two getCurrentIntervalMs() assertions to match the new formula (level 1 → 1320 ms; floor-clamped test now needs 57 iterations instead of 81).