Extends VCR speed cycle to `[0.25, 0.5, 1, 2, 4, 8]` so users can watch
live paths in slow motion.
## Changes
- `vcrSpeedCycle()`: speed array extended to include `¼x` and `½x`;
saves preference to `localStorage('live-vcr-speed')`
- `speedLabel()`: new helper returning `¼x` / `½x` for sub-1x, used in
the speed button
- `drawAnimatedLine`: step interval scales with speed (`33 / VCR.speed`)
- `drawMatrixLine`: `DURATION_MS` scales with speed (`1100 / VCR.speed`)
- Speed preference restored from localStorage on page load
## Tests
3 new unit tests; 72 pass, 0 regressions.
Closes#771 (M1 of 3)
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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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