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/pid/__init__.py

@@ -1,5 +1,4 @@
 from components.pid.pid import *
 from components.pid.kalman import *
-from components.pid.tc_constants import *
 
 from components.pid.pid_factory import PidFactory