feat: add flammable wood that burns from adjacent fire

[chowjiaming]

Summary

• Add Cell::Wood: static like stone, brown, eraser-only overwrite.
• Fire ignites cardinal wood neighbors with p=0.1 per tick and stays put while fuel remains, so a flame does not float off a wall.
• Also includes unmerged work from 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, 13 unit tests, wasm-pack)
• Paint a wood block, touch it with fire, confirm it catches and burns through
• Confirm fire still rises in open air and wood does not fall
• Confirm sand/water cannot paint over wood (eraser can)

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