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

client/brewpi_gui.py

@@ -151,9 +151,9 @@ class SudForecastPlot(FigureCanvasQTAgg):
 		for step in schedule:
 			ramp = step.get('ramp')
 			hold = step.get('hold')
-			if ramp is not None:
+			target = step.get('temperature')
+			if ramp is not None and target is not None:
 				rate = ramp.get('rate', 0)
-				target = step['temperature']
 				if rate > 0:
 					t.append(t[-1] + abs(target - theta[-1]) / rate * 60.0)
 					theta.append(target)