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>

server/brewpi.py

@@ -110,6 +110,7 @@ if __name__ == '__main__':
 	# the kettle at startup (e.g. strike water pre-filled). A loaded sud's
 	# own apply_plant_params() will override this on the first step.
 	startup_water = config.get('Pot', {}).get('water_mass', 0)
+	startup_params = None
 	if startup_water > 0:
 		startup_params = sud.derive_plant_params(0, startup_water)
 		pot.set_plant_params(startup_params)
@@ -131,8 +132,10 @@ if __name__ == '__main__':
 
 	# Continuous server-session log - records from startup to shutdown
 	# regardless of Sud state; written to logs/log_{date_time}.json on exit.
-	server_log_task = ServerLogTask(tc, heater, heater_task, DT_TASK, args.logdir, config=config)
+	server_log_task = ServerLogTask(tc, heater, heater_task, DT_TASK, args.logdir, config=config, dt=DT)
 	sud_task.set_on_plant_params(server_log_task.log_plant_params)
+	if startup_params is not None:
+		server_log_task.log_plant_params(0, startup_params)
 	taskmgr.add(server_log_task)
 
 	# Assign data flow