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described the Kalman-filter/heat-diffusion design that's since been
replaced by the delay-line Pot model and Kalman-free Smith predictor.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>

components/plant/TODO.md

@@ -1,51 +1,49 @@
 # components/plant design backlog
 
 Findings from reviewing `pot.py`'s water-bath model against real-plant
-behavior (sim/real control mismatch reported by user). Not yet acted on.
+behavior (sim/real control mismatch reported by user), refreshed against
+the current `delay`-line rewrite of `Pot`.
 
-- [ ] **Heat-loss term has wrong units.** `pot.py:37`:
-  `leak = 1/self.M * self.L * (self.theta_amb - self.temp)/self.theta_amb`.
-  The power term right above it is `power/(self.M * self.C)` (W /
-  (J/K) -> K/s). `leak` should follow the same pattern —
-  `self.L * (self.theta_amb - self.temp) / (self.M * self.C)` — but
-  instead it's missing `/C` and divides by `theta_amb` (~20) instead,
-  which isn't a physically meaningful normalization. With
-  `C=4190` this suppresses ambient heat loss by ~3-4 orders of
-  magnitude versus what the power term implies, so the simulated pot
-  barely cools at all. The Smith predictor's internal model is also a
-  `Pot` instance with the identical bug, so this never showed up in
-  sim-vs-sim testing — only against the real plant. Likely the
-  primary cause of the sim/real control mismatch.
+- [x] **Heat-loss term has wrong units.** Fixed by the `Pot` rewrite:
+  `process()` now computes `p_loss = self.L * self.M * (self.temp -
+  self.theta_amb)` in Watts and applies `self.temp += (self.delay.get()
+  - p_loss) / (self.M * self.C) * self.dt` — the same `/(M*C)`
+  normalization for both the gain and loss terms, instead of the old
+  mismatched `leak = 1/self.M * self.L * (...)/ self.theta_amb`.
 
-- [ ] **`kn` is loaded but never applied.** `pot.py:18` reads
-  `params['kn']` but nothing in `process()`/`get_temperature()` uses
-  it. Git history shows it used to add Gaussian sensor noise
-  (`self.kn * np.random.normal(...)`) to the output, since commented
-  out and then dropped during the heat-diffusion refactor. Sim is
-  currently fully deterministic/noise-free, making the Kalman filter
-  and PID derivative term look better-behaved in sim than they will
-  against a real noisy sensor. Re-enable as measurement noise (and
-  decide if process noise is also wanted).
+- [x] **Thermal lag modeled as single-pole lag, not transport delay.**
+  Fixed: `Pot` no longer uses `HeatDiffusion`'s `alpha=dt/Td`
+  exponential smoothing (that module was deleted). It now delays the
+  input power through `plant/delay.py`'s `Delay` (true dead time)
+  before integrating it, matching what this item recommended.
 
-- [ ] **`theta_amb` is hardcoded, not configured.** `brewpi.py:40`
-  passes a literal `20` instead of reading it from config or a live
-  ambient sensor. Real ambient temperature drifts and is never
-  modeled as a disturbance, so the controller's robustness to it is
-  untested.
+- [x] **`kn` is loaded but never applied.** Resolved differently than
+  originally proposed: `kn` was removed from `Pot` entirely rather than
+  wired up as plant-side measurement noise. Sensor noise is now
+  modeled one layer up, in `TempSensorSim` (`temp_offset`/`variance`,
+  wired from `brewpi.py`'s `TempSensorFactory.create(..., temp_offset=
+  -0.15, variance=0.01)`), not in the plant model. Note: `config.json.templ`
+  and `config.json.sim` still carry stale `Model.kn`/`Plant.kn` keys
+  that nothing reads anymore — harmless but worth deleting (see
+  `components/pid/TODO.md`'s config-validation item).
 
-- [ ] **Thermal lag modeled as single-pole lag, not transport delay.**
-  `HeatDiffusion` (`heat_diffusion.py`) implements `alpha=dt/Td`
-  exponential smoothing, not true dead time. There's already a
-  `Delay` class (`plant/delay.py`) for pure transport delay, dropped
-  in favor of `HeatDiffusion` in commit `16cb3d3`. If the real pot's
-  heater-to-sensor coupling behaves more like dead time (heat must
-  propagate through the water before reaching the sensor) than a
-  single lag, `Td` won't represent the same dynamic in sim vs. reality,
-  and the Smith predictor's delay compensation (which assumes `Td`
-  matches the real plant) will be off.
+- [ ] **`theta_amb` is hardcoded, not configured.** Still true —
+  `brewpi.py`: `pot = Pot(DT, pot_params, 20)` passes a literal `20`
+  instead of reading it from config or a live ambient sensor. Real
+  ambient temperature drifts and is never modeled as a disturbance, so
+  the controller's robustness to it is untested.
 
-- [ ] **No actuator/process realism.** No heater power saturation
-  enforced inside `Pot` itself (handled ad hoc in test harnesses), no
-  evaporation/lid heat loss, no varying thermal mass `M` as volume
+- [ ] **No actuator/process realism.** Partially improved: `process()`
+  now clamps `self.temp` to `min(100, ...)` (won't simulate past
+  boiling), but there's still no heater power saturation enforced
+  inside `Pot` itself (handled ad hoc in demo harnesses), no
+  evaporation/lid heat loss, and no varying thermal mass `M` as volume
   changes (e.g. boil-off, adding water/grain) — `M` is a fixed
-  constant for the whole simulation.
+  constant for the whole simulation.
+
+- [ ] **Debug `print()`s left in `Pot.process()`.** Every call prints
+  `p_loss`, `theta_amb`, and `temp` unconditionally — this runs once
+  per simulated/real tick (e.g. every `Controller.dt`), spamming stdout
+  in both the server and every demo script. Looks like leftover
+  debugging from the heat-loss-units fix; remove or gate behind a
+  debug flag.