Consolidate States/DEFAULT_THRESHOLDS into temp_controller_base.py

tc_constants.py only held States and DEFAULT_THRESHOLDS, both used
exclusively by temp_controller_base.py and its subclasses; fold them
into temp_controller_base.py directly and drop the now-empty module.
Also drop heat_diffusion.py, unused since pot.py switched to the
delay-line model, and the matching dead import in pot.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/plant/pot.py

@@ -1,5 +1,4 @@
 from components.aplant import APlant
-from components.plant.heat_diffusion import HeatDiffusion
 from components.plant import delay
 
 class Pot(APlant):