fix: embed dt and startup plant params in server log; open-loop replay_sim

server/brewpi.py: pass physics DT to ServerLogTask (not wall-clock DT_TASK)
so logs carry the correct integration timestep; log startup plant params as
the first PlantParams event so replay_sim can reconstruct them even when no
Sud is ever loaded (previously fell back to hardcoded defaults, causing ~3°C
temperature offset in replay).

tasks/server_log.py: accept and embed dt in log JSON; switch
log_plant_params() to wall-clock monotonic time (same base as sample t),
since the elapsed argument was simulated time and not comparable.

utils/replay_sim.py: switch to open-loop replay (plant driven by logged
power_eff, not sim controller output); use PidFactory so Smith vs Normal
controller type is honoured; set Td=0 in replay plant for Smith mode (logged
temp_ist is already Smith-corrected); feed Smith predictor's internal model
with real power; apply embedded PlantParams mid-run; prefer embedded dt over
sample-based fallback; clamp sim power ≥ 0; replace meaningless sim-power
trace in power subplot with power_set vs power_eff from the log.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tasks/server_log.py

@@ -16,20 +16,21 @@ class ServerLogTask(ATask):
 	Sud state — useful for verifying controller and heater behaviour
 	outside of a scheduled brew."""
 
-	def __init__(self, tc: APid, heater: AHeater, heater_task, interval, path='./logs', config=None):
+	def __init__(self, tc: APid, heater: AHeater, heater_task, interval, path='./logs', config=None, dt=None):
 		ATask.__init__(self, interval)
 		self.tc = tc
 		self.heater = heater
 		self.heater_task = heater_task
 		self.path = path
 		self.config = config
+		self.dt = dt
 		self._samples = []
 		self._param_events = []
 		self._t0 = time.monotonic()
 		self._run_id = time.strftime("%Y%m%dT%H%M%S", time.localtime())
 
 	def log_plant_params(self, elapsed, params):
-		self._param_events.append({'t': elapsed, 'params': params})
+		self._param_events.append({'t': time.monotonic() - self._t0, 'params': params})
 
 	async def on_process(self):
 		while True:
@@ -53,6 +54,8 @@ class ServerLogTask(ATask):
 		filename = 'log_{}.json'.format(self._run_id)
 		path = os.path.join(self.path, filename)
 		log_data = {'Name': ''}
+		if self.dt is not None:
+			log_data['dt'] = self.dt
 		if self.config is not None:
 			log_data['Config'] = self.config
 		if self._param_events: