[chowjiaming]

Summary

• Density table: gases 0, oil 1, water 2, sand/gunpowder 3, stone/wood/fans/ice 255. Sand still sinks through water; oil floats.
• Oil slides like water, paints at room heat 50, ignites from cardinal fire or heat 60, does not boil.
• Ice is locked and blocks air. Melts to water at heat 25; water freezes below 15. Stone/fans do not take heat so ponds against walls stay liquid.

Test plan

• just verify
• Oil ends above water in a sealed column
• Oil ignites from adjacent fire
• Painted water does not freeze in 16 ticks
• Ice under fire melts within ~32 ticks
• from_u8(13..=14) round-trips; sand_sinks_through_water still green

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