Add design-flaw backlog for components/plant

Review of pot.py found a likely root cause for sim-vs-real control
mismatches: the heat-loss term carries the wrong units, masking
ambient cooling in simulation. Also notes the dropped sensor-noise
term, hardcoded ambient temp, and lag-vs-dead-time modeling gap.

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

components/plant/TODO.md

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+# 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.
+
+- [ ] **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.
+
+- [ ] **`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).
+
+- [ ] **`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.
+
+- [ ] **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.
+
+- [ ] **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
+  changes (e.g. boil-off, adding water/grain) — `M` is a fixed
+  constant for the whole simulation.