Share heat-source detection across machines

[Alatyami]

Summary

• centralize heat-source classification in HeatSourceUtils
• make Mixing Vat honor the same growthcraft_cellar:heat_sources block tag as the Cellar machines
• reuse the helper from Brew Kettle, Roaster, and Culture Jar to remove duplicated classification logic
• preserve each machine's existing search geometry: directly below for Vat/Kettle/Roaster and radius two for Culture Jar

Root cause

Mixing Vat maintained a separate hard-coded heat check. It recognized several vanilla blocks but ignored datapack additions to the established heat-source tag and did not share the lava fallback used by the Cellar machines.

Validation

• ./gradlew.bat compileJava -x createMinecraftArtifacts
• ./gradlew.bat test -x createMinecraftArtifacts
• git diff --check
• manually added minecraft:diamond_block to growthcraft_cellar:heat_sources through a temporary datapack and confirmed the machines detected it as expected

Closes #196