Refresh README and design-backlog docs against current code

Both TODO.md backlogs and the README's architecture section still
described the Kalman-filter/heat-diffusion design that's since been
replaced by the delay-line Pot model and Kalman-free Smith predictor.
Check off the items that rewrite already fixed (heat-loss units,
transport delay vs. single-pole lag, three-Kalman-tuning), note how
the kn/sensor-noise item was resolved differently than proposed, and
add newly-spotted issues: brewpi.py wiring set_model_power
unconditionally (breaks pid_type "Normal"), kalman.py being dead code
in production, stale Kalman/kn keys in the config templates, and
debug print()s left in Pot.process().

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

README.md

@@ -6,8 +6,8 @@ rate) according to a configurable mash schedule, drives a heater and stirrer,
 and exposes live control/telemetry over a WebSocket so a desktop GUI (or any
 other client) can monitor and steer the brew.
 
-The project began as a simulation/control-theory playground (Kalman filter +
-Smith-predictor temperature control, pot heat-diffusion model, see
+The project began as a simulation/control-theory playground (Smith-predictor
+temperature control, pot transport-delay model, see
 [`docs/NonLinMPC.pdf`](docs/NonLinMPC.pdf)) and has grown real-hardware
 backends for an induction hob and an RTD temperature probe.
 
@@ -26,10 +26,13 @@ client/brewpi_gui.py        PyQt5 desktop client (brewpi.ui) that connects
                              and switch the heater/stirrer on or off.
 
 components/                 Pluggable building blocks behind factories:
-  pid/                         temperature controllers (PID, Smith
-                                predictor + Kalman filter) and the math
-                                model used for prediction
-  plant/                       pot heat-diffusion model used in simulation
+  pid/                         temperature controllers: plain PID
+                                ("Normal") or PID + Smith predictor
+                                ("Smith", runs two internal pot models —
+                                one with the plant's transport delay, one
+                                without — to compensate for the dead time)
+  plant/                       pot thermal model (transport-delay line)
+                                used in simulation
   sensor/                      temperature sensors: simulated, or a real
                                 MAX31865 RTD amplifier over SPI
   actor/                       heater and stirrer drivers: simulated, a