feat: add smoke from burned-out fire

[chowjiaming]

Summary

• Cell::Smoke (id 12) is a gas with paint heat 80. Isolated fire TTL/flicker death becomes smoke (heat kept); water quench still goes to Empty.
• Smoke uses the same air-push as steam but skips rise every other tick, cools 1/tick, and disperses below heat 10.
• Palette swatch and mid-gray renderer color.

Test plan

• just verify
• Isolated fire becomes smoke then Empty
• Fire next to water does not spawn smoke
• Over 8 ticks smoke stays lower than steam
• from_u8(12) round-trips

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