Make Sud steps ramp based on the actual temperature gap, not a 'ramp' key

Previously, whether a schedule step ramped at all was decided purely by
the presence of a 'ramp' key (components/sud.py's _advance()) - a
hold-only step jumped straight into its hold countdown, assuming the
plant was already at temperature. Now every step ramps toward
'temperature' first, for as long as the controller's own gap-tracking
FSM (TempControllerBase, already distinguishing HEAT/COOL/HOLD) says
the gap actually warrants it - via the new is_holding() (on APid and
TempControllerBase), which replaces a separate, redundant
TEMP_REACHED_TOLERANCE constant duplicated across tasks/sud.py,
components/sud_forecast.py, and scripts/demos/sud/demo_sud.py.

set_theta_soll() now recomputes the FSM eagerly so is_holding() can't
read stale HOLD for a tick after a much-further-away target is pushed.

components/sud.py's _build_step() always synthesizes a 'ramp' block
from default.step.ramp (so 'rate' is always available), but leaves
'temperature' undefaulted - every real sude/*.json's
default.step.temperature is inert template filler, and defaulting it
would send hold-only steps chasing 0 degrees. SudTask/
SudForecastEstimator/the demo only push a new theta_soll/heatrate_soll
when a step actually specifies its own temperature; otherwise the
controller keeps whatever the previous step left running.

Also fixes a related crash this surfaced: SudTask.remaining_schedule()'s
synthetic mid-hold step dropped 'ramp' to signal "don't re-ramp" - now
that every step always carries a 'ramp' dict, dropping it left 'rate'
missing the moment the synthetic step was re-resolved by
SudForecastEstimator. Drops 'temperature' instead, which is what
actually signals "no new target" under the new model.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LhiQe64F74uHV8jzuoSa5K

scripts/demos/sud/demo_sud.py

@@ -7,9 +7,6 @@ from components.plant.pot import Pot
 from components.actor.stirrersim import StirrerSim
 from components.actor.heater_sim import HeaterSim
 
-# Mirrors tasks/sud.py's SudTask, but driven synchronously for a demo run.
-TEMP_REACHED_TOLERANCE = 0.2
-
 # A real user would click "Confirm" in the GUI; auto-confirm after this many
 # simulated seconds so the demo can run to completion unattended.
 WAIT_USER_AUTO_CONFIRM_S = 30.0
@@ -85,11 +82,11 @@ if __name__ == '__main__':
 		# sud.state tells us which phase is currently active.
 		if step is not None:
 			ramping = sud.state == SudState.RAMPING
-			phase = step['ramp'] if ramping and 'ramp' in step else step.get('hold', {})
+			phase = step['ramp'] if ramping else step.get('hold', {})
 
 			print(f"Step {step['number']}: {step['descr']}")
 			apply_plant_params(step)
-			if ramping and 'ramp' in step:
+			if ramping and step['temperature'] is not None:
 				ctrl.set_theta_soll(step['temperature'])
 				ctrl.set_heatrate_soll(step['ramp']['rate'])
 			apply_stirrer(phase)
@@ -138,7 +135,7 @@ if __name__ == '__main__':
 		stirrer.process()
 
 		if sud.state == SudState.RAMPING:
-			if abs(ctrl.get_theta_ist() - ctrl.get_theta_soll_set()) < TEMP_REACHED_TOLERANCE:
+			if ctrl.is_holding():
 				sud.temp_reached()
 
 		if sud.state == SudState.WAIT_USER: