Derive Pot's L/Td from the Sud doc instead of a fixed server default

Sud now parses top-level L/Td fields from sude/*.json (defaulting to 0.2/30,
matching the old hardcoded server constants), and derive_plant_params()
returns them alongside M/C - constant for the whole brew, unlike M/C which
vary per step with grain/water mass. All sude/*.json docs gain explicit
L/Td fields.

Callers (tasks/sud.py, SudForecastEstimator, demo_sud.py) switch from the
narrow set_thermal_params(M, C)/set_model_params(M, C) to the full
set_plant_params(params)/set_model_plant_params(params), since
derive_plant_params() now always returns all four keys; both narrow setters
are removed as dead code.

Caught along the way: Pot.set_plant_params() unconditionally rebuilt the
Delay ring buffer, which was harmless when only ever called once at
construction - but now running on every Sud step change, it was wiping
in-flight delayed power at each step boundary. Fixed to only rebuild when
Td actually changes; verified this restores the exact original forecast
result for sud_0010.json.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DkkuG48uHFCGKe6dPSERFk

scripts/demos/sud/demo_sud.py

@@ -42,11 +42,7 @@ if __name__ == '__main__':
 		sud.load(json.load(f))
 
 	first_step = sud.schedule[0]
-	plant_params = {
-		**sud.derive_plant_params(first_step['grain_mass'], first_step['water_mass']),
-		"L"			: 0.2,
-		"Td"		: 60
-	}
+	plant_params = sud.derive_plant_params(first_step['grain_mass'], first_step['water_mass'])
 
 	ctrl = TempController(dt)
 	ctrl.set_params(ctrl_params)
@@ -79,8 +75,8 @@ if __name__ == '__main__':
 
 	def apply_plant_params(step):
 		params = sud.derive_plant_params(step['grain_mass'], step['water_mass'])
-		plant.set_thermal_params(params['M'], params['C'])
-		ctrl.set_model_params(params['M'], params['C'])
+		plant.set_plant_params(params)
+		ctrl.set_model_plant_params(params)
 
 	def on_step_changed(step):
 		# A step may carry both 'ramp' and 'hold' (ramp to temp, then hold);