Fix LM-919 review follow-ups

[brsbl]

Human comments

What was wrong

The LM-919 review (#42) left a list of non-blocking issues: start/stop edge cases, Firefox and background-tab glitches, undo and keyboard rough edges, a silent synth at SPEED 1/4, unthrottled share-link rewrites, and stale docs and assets.

What changed

• Holding the first key at SPEED 1/4 now fills a rhythm that plays.
• Re-clicking a lit option, or Reset on an empty grid, no longer adds an undo step.
• LED option groups take arrow keys, and each group is one Tab stop.
• Undo keeps keyboard focus after the last undo.
• Escape cancels a typed tempo; Enter still applies it.
• Trackpad swipes move knobs and faders a few notches instead of slamming them.
• Dragging a knob or fader no longer selects page text.
• Screen readers hear Start/Stop once and the octave-shifted key names.
• The share link trails edits by 250 ms and survives browser rewrite limits.
• Stop now cancels hits queued in the next 100 ms.
• Double-clicking Start no longer leaves a stray animation loop.
• Choked kicks and hats stop their sources instead of running silently.
• A stalled browser tab leaves a gap instead of a burst of stacked hits.
• A held chord stops droning when no synth step can fire.
• Firefox keeps the synth's decay when a note is released early.
• The unused 808 samples (11 MB) are gone, and README, docs, and comments match the code.

How you verified

• Remote CI passed lint, typecheck, test, and build on 7efa5a7. Run
• New tests cover undo no-ops, link tempo parsing, rate-aware fill, the envelope, and clock catch-up.
• Chrome for Testing QA passed: arrow keys, Undo focus, Escape, wheel, Start/Stop, and link timing.

Screenshots

Before: 7e12648 · After: 7efa5a7 · Captured at 2× in Chrome for Testing

Screen / state	Viewport	Before	After
Synth / first key held at SPEED 1/4	Desktop · 1440×900

BB-Thread-ID: thr_d58s3peww2

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