Add random-dot motion web stimulus

[zh1peng]

What changed

• adds a typed random_dot_motion stimulus specification
• renders deterministic, limited-lifetime coherent random dots on canvas
• cancels animation frames during stage cleanup
• adds T/H000076 alignment and outcome tests

Why

H000076 needs a browser-native random-dot kinematogram that preserves the canonical dot count, aperture, speed, lifetime, direction, coherence, and seed semantics.

Validation

• npm test -- --run: 74 test files, 141 tests passed
• npm run build: production build passed
• real-browser validation: correct and incorrect direction paths, confidence recoding, raw/reduced export panels, and zero console warnings/errors

The current shared workspace later gained unrelated H000077/H000078 authoring files that make the repository-wide TypeScript command fail outside this PR's files; those files are intentionally excluded from this branch.