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described the Kalman-filter/heat-diffusion design that's since been
replaced by the delay-line Pot model and Kalman-free Smith predictor.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

@@ -1,15 +1,26 @@
 # components/pid design backlog
 
-Findings from a design review, not yet acted on. See git history on
-`claude_refactor` for items already fixed (Smith predictor's delay
-correction enabled, the two TempController classes deduplicated into
-`temp_controller_base.py`).
+Findings from a design review, refreshed against the current state of
+`master` after the Kalman-filter removal and Smith-predictor rewrite (see
+git history for `temp_controller.py`/`temp_controller_smith.py`).
 
 - [x] **FSM thresholds aren't configurable.** Moved into an optional
   `TempCtrl.Thresholds` config section (`HoldIdle`, `HoldHeat`,
   `IdleHeat`, `IdleHold`, `HeatHold`, `HeatIdle`), merged over
-  `DEFAULT_THRESHOLDS` in `tc_constants.py` so existing configs without
-  the section keep working unchanged.
+  `DEFAULT_THRESHOLDS` (now in `temp_controller_base.py`, formerly
+  `tc_constants.py`) so existing configs without the section keep
+  working unchanged.
+
+- [x] **matplotlib imported at module level in production code.**
+  `temp_controller.py`, `temp_controller_smith.py`, and `kalman.py`
+  used to `from matplotlib.pyplot import ...` just to support their
+  `__main__` self-test plots. The plotting demos (and `kalman_eval.py`)
+  moved to `scripts/demos/pid/demo_*.py`; production modules no longer
+  import matplotlib.
+
+- [x] **Three Kalman filters share one tuning.** No longer applicable:
+  `temp_controller_smith.py` was rewritten to drop Kalman filtering
+  entirely (see below), so there's no shared tuning to split anymore.
 
 - [ ] **`Pid.scale()` is a gain-scheduling hack, not anti-windup.**
   `pid.py`'s `scale(k)` multiplies every gain by `1/heatrate_soll_set`
@@ -18,30 +29,41 @@ correction enabled, the two TempController classes deduplicated into
   a few lines below. `self.k` also never resets in `reset()`, so a
   stale scale factor can survive a state-transition reset. Consider
   moving this scheduling logic out of the generic `Pid` class and into
-  the temp controller, and resetting `k` in `reset()`.
+  the temp controller, and resetting `k` in `reset()`. Still present.
 
-- [ ] **Three Kalman filters share one tuning.** In
-  `temp_controller_smith.py`, `kalman_model`, `kalman_model_delay`,
-  and `kalman_plant` are all constructed with the same
-  `params['Kalman']` despite tracking signals with different noise
-  characteristics. Allow independent `var_P`/`var_Q`/`var_R` per
-  filter in config.
+- [ ] **No automated tests.** `tests/components/pid/` was added once
+  (stdlib `unittest`, no pytest) covering FSM transitions, anti-windup
+  clamping, and Kalman convergence, but was removed again before the
+  Kalman-filter removal/Smith rewrite, so none of the current
+  `temp_controller*.py` behavior (backward-difference + low-pass
+  filtered heat rate, the two-model Smith correction) has test
+  coverage. This drives a physical heater — worth re-adding.
 
-- [ ] **matplotlib imported at module level in production code.**
-  `temp_controller.py`, `temp_controller_smith.py`, and `kalman.py`
-  all `from matplotlib.pyplot import ...` just to support their
-  `__main__` self-test plots, which couples the live server's import
-  graph (and its dependency list) to a GUI plotting library it never
-  uses at runtime. Move the plotting demos into separate scripts.
-
-- [ ] **No automated tests.** The `__main__` blocks (and
-  `kalman_eval.py`) are manual, eyeballed-plot harnesses. Add
-  regression tests for FSM transitions, anti-windup clamping, and
-  Kalman convergence — this code drives a physical heater.
+- [ ] **`set_model_power` isn't defined on every controller, but
+  `brewpi.py` wires it unconditionally.** `brewpi.py` always does
+  `heater.set_on_changed("power_set", tc.set_model_power)` regardless
+  of `Controller.pid_type`. Only `temp_controller_smith.py` (the Smith
+  predictor) defines `set_model_power`; `temp_controller.py` (`"Normal"`)
+  does not, so starting the server with `"pid_type": "Normal"` crashes
+  at wiring time with `AttributeError: 'TempController' object has no
+  attribute 'set_model_power'`. Either add a no-op `set_model_power` to
+  `TempControllerBase`, or only wire it when the configured controller
+  actually exposes a model to feed.
 
 - [ ] **Config access is unchecked `dict[key]` everywhere.**
-  `params['Kalman']`, `model_params['gain']`, etc., throughout, with
-  no schema validation at load time. We already hit this bug class
-  once (`config.json.sim`'s `gain`/`Model` nesting mismatch). A small
+  `params['Hold']`, `model_params['gain']`, etc., throughout, with no
+  schema validation at load time. We already hit this bug class once
+  (`config.json.sim`'s `gain`/`Model` nesting mismatch). A small
   schema/dataclass validation layer at config-load would surface
-  errors immediately instead of mid-`__init__`.
+  errors immediately instead of mid-`__init__` — and would have caught
+  the dead `TempCtrl.Kalman` / `Model.kn` / `Plant.kn` keys still
+  sitting in `config.json.templ` and `config.json.sim` (harmless today
+  since extra keys are just ignored, but nothing reads them anymore
+  after the Kalman/`kn` removal — see `components/plant/TODO.md`).
+
+- [ ] **`kalman.py` is now dead code in production.** Neither
+  `temp_controller.py` nor `temp_controller_smith.py` uses `Kalman`
+  anymore; the only remaining references are
+  `scripts/demos/pid/demo_kalman.py` and `demo_kalman_eval.py`. Decide
+  whether to keep it as a documented standalone filtering example or
+  delete it along with the demos.