Add a simulation-based Sud forecast, computed server-side

New components/sud_forecast.py: SudForecastEstimator simulates a whole
schedule with the same kind of plant/controller (and the same
configured params - ambient, plant params, pid_type, TempCtrl gains,
heater max power) the server uses for real, by driving a throwaway
Sud/Pot/controller trio through it exactly as tasks/sud.py's SudTask
would. It's pure CPU-bound iteration (no real time/IO), so a multi-
hour brew simulates in well under a second.

tasks/sud.py: SudTask now takes an optional forecast_estimator and
sends its result ({'Forecast': {'T': ..., 'Theta': ...}}) on the Sud
channel after every successful Save/Load, run in a worker thread via
run_in_executor so the ~100-500ms simulation doesn't stall the other
tasks. server/brewpi.py constructs one with the real server's config
and wires it in; AmbientTemp changes update it too.

client/brewpi_gui.py: the quick naive show_schedule() estimate (drawn
immediately on Load, before the server's simulation finishes) is now
superseded by show_precomputed_course() once the Forecast message
arrives - only while not actively running, so it doesn't fight the
dynamic re-anchored view.

Verified: matches a standalone run of the estimator (177 min for
sude/sud_0010.json) and replaces the old naive 164 min estimate live
within a couple seconds of loading.

components/sud_forecast.py

@@ -0,0 +1,98 @@
+from components.plant import Pot
+from components.pid import PidFactory
+from components.sud import Sud, SudState
+
+# Real schedules need real time to spin up each ramp and settle within this
+# tolerance before "reached" fires - matching tasks/sud.py's
+# TEMP_REACHED_TOLERANCE exactly is what makes this simulation's predicted
+# duration line up with the real run's.
+TEMP_REACHED_TOLERANCE = 0.2
+
+# Safety cap so a schedule whose target a step can never actually reach
+# (e.g. a "hold" colder than ambient with no active cooling) can't hang the
+# simulation forever - the estimate is simply cut off there.
+MAX_TICKS = 200000
+
+
+class SudForecastEstimator:
+	"""Predicts how long a Sud schedule will actually take by simulating it
+	with the same machinery (and params) the real server's brewpi.py wires
+	up - a fresh Pot and temperature controller of the configured pid_type,
+	driven through the schedule exactly as tasks/sud.py's SudTask would.
+
+	This is deliberately independent of wall-clock/asyncio time: it just
+	iterates dt-sized ticks as fast as the CPU allows (a multi-hour brew
+	simulates in well under a second), so it can be run synchronously
+	whenever a client needs an estimate - the naive "abs(delta)/rate" model
+	the GUI used to compute itself has no way to see the real PID cascade's
+	spin-up/settling lag, which is exactly why its estimate drifted so far
+	from reality (see README.md's "Forecast vs. actual duration")."""
+
+	def __init__(self, dt, theta_amb, plant_params, pid_type, tempctrl_params, heater_max_power):
+		self.dt = dt
+		self.theta_amb = theta_amb
+		self.plant_params = plant_params
+		self.pid_type = pid_type
+		self.tempctrl_params = tempctrl_params
+		self.heater_max_power = heater_max_power
+
+	def set_ambient_temperature(self, theta_amb):
+		self.theta_amb = theta_amb
+
+	def estimate(self, doc, start_theta=None):
+		"""Returns (t, theta): parallel lists of elapsed simulated seconds
+		and temperature, one point per tick, for the given sud.json
+		document. start_theta defaults to the configured ambient
+		temperature - i.e. a cold start, same as the GUI's static
+		estimate."""
+		if start_theta is None:
+			start_theta = self.theta_amb
+
+		sud = Sud()
+		if not sud.load(doc) or not sud.schedule:
+			return [0.0], [start_theta]
+
+		pot = Pot(self.dt, self.plant_params, self.theta_amb)
+		pot.initial(start_theta)
+		tc = PidFactory.create(self.pid_type, self.dt, self.tempctrl_params, self.plant_params, theta_amb=self.theta_amb)
+		tc.set_enabled(True)
+		tc.set_theta_ist(pot.get_temperature())
+
+		def on_step_changed(step):
+			if step is None:
+				return
+			params = sud.derive_plant_params(step.get('grain_mass', 0), step.get('water_mass', 0))
+			pot.set_thermal_params(params['M'], params['C'])
+			if hasattr(tc, 'set_model_params'):
+				tc.set_model_params(params['M'], params['C'])
+			ramp = step.get('ramp')
+			if sud.state == SudState.RAMPING and ramp is not None:
+				tc.set_theta_soll(step['temperature'])
+				tc.set_heatrate_soll(ramp['rate'])
+		sud.set_on_changed('step', on_step_changed)
+
+		t = [0.0]
+		theta = [pot.get_temperature()]
+
+		sud.start()
+		ticks = 0
+		while sud.state != SudState.DONE and ticks < MAX_TICKS:
+			pot.process()
+			tc.set_theta_ist(pot.get_temperature())
+			tc.process()
+			pot.set_power(max(0, self.heater_max_power * tc.get_power()))
+
+			if sud.state == SudState.RAMPING:
+				if abs(tc.get_theta_ist() - tc.get_theta_soll_set()) < TEMP_REACHED_TOLERANCE:
+					sud.temp_reached()
+			elif sud.state == SudState.WAIT_USER:
+				# A real user's confirm time isn't predictable - count it
+				# as instant for estimation purposes.
+				sud.confirm()
+			sud.tick(self.dt)
+
+			t.append(t[-1] + self.dt)
+			theta.append(pot.get_temperature())
+			ticks += 1
+
+		return t, theta