feat: add fire that rises, flickers, and burns out

[chowjiaming]

Summary

• Add Cell::Fire: a gas that rises, carries a per-cell TTL, flicker-despawns near the end of its life, and ignites nothing yet.
• Orange/red color ramp from remaining lifetime, plus a Fire palette button.
• Also includes the unmerged floor-water fix from fix: let floor water slide and lock in sand/water feel #1 (step() now visits the last row so puddles on the world floor can spread).

Test plan

• just verify (fmt, clippy, 11 unit tests, wasm-pack)
• Paint Fire and confirm it rises, flickers, and burns out without falling
• Paint water on the canvas floor and confirm it spreads
• Pour sand and confirm a stable ~45° pile

[cursor]

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