feat: add ambient heat, pressure boil, and glass

[chowjiaming]

Summary

• Coarse ah on the air grid (Empty pixel heat stays 0). Gases dump heat into air; solids take heat from hotter air. Hot air above room temperature gets a Boussinesq upward vy bias.
• Boil point is WATER_BOIL + max(pv,0)/40. Steam still condenses in still air.
• Sand at heat 180 becomes locked Glass (id 15), dirty green-gray, palette button.

Test plan

• just verify
• High-pv water needs more heat to boil than still water
• Isolated steam still condenses to water
• Sand next to trapped fire becomes glass; add_heat(180) is one-way
• Empty heat() stays 0 next to fire
• from_u8(15) round-trips; from_u8(16) is None

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