feat: add brush slider, pause/step, and palette swatches

[chowjiaming]

Summary

• Palette buttons get color swatches; brush radius is a 1–8 slider.
• Pause/Resume and Step freeze the sim and advance one tick (redraw() does not step). Space toggles pause, . single-steps.
• Also includes unmerged work from perf: skip idle 16×16 chunks in World::step #4 (chunks), feat: add flammable wood that burns from adjacent fire #3 (wood), feat: add fire that rises, flickers, and burns out #2 (fire), and fix: let floor water slide and lock in sand/water feel #1 (floor-water spread).

Test plan

• just verify (fmt, clippy, 15 unit tests, wasm-pack)
• Pause, paint, confirm cells appear without the world advancing
• Step once and confirm a single tick
• Change brush size and confirm the stamp radius
• Confirm FPS still ~60 at 320×200 in a release serve

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