Add Grid Heat API

[KingParmenides]

Fixes #1192.

Summary

• expose grid heat through a copyable GridHeatApi wrapper with AddHeat, SetHeat, heat query methods, and API-owned capacity/dissipation/generation controls
• make the heat manager track already-loaded grids and handle API-created heat state outside fusion assemblies
• let the existing heat HUD render API-managed grid heat and move/hide the HUD through the API
• add a short usage guide for other mods

Verification

• git diff --check

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