feat: add lookahead scheduler

[britle20]

Summary

Closes #5.

Adds a tick-based lookahead scheduler for 1-bar loop playback and exposes sample loop transport APIs from the browser audio engine.

Changes

• Added PPQ 480 tick/time conversion utilities and tests.
• Added a React-independent lookahead scheduler with loop boundary tests.
• Added startSampleLoop, stopLoop, and transport snapshot APIs to the browser audio engine.
• Moved unit tests under tests/unit/ and added tsconfig.test.json so test files are typechecked.
• Updated audio engine/testing docs, feature status, and Active Issue Order.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build

Screenshots

Not applicable. This is audio scheduling infrastructure without UI changes.

Notes / Limitations

• Manual browser audio listening was not performed because no UI caller exists for this scheduler yet.
• stopLoop() clears future scheduler windows, but Web Audio events already submitted inside the short schedule-ahead window may still fire.

Follow-up

• Wire drum sequencer clip events into startSampleLoop() in a later feature.