Add design-flaw backlog for components/pid

Tracks open issues from a review that weren't fixed as part of this
branch's Smith-predictor and dedup work: non-configurable FSM
thresholds, Pid.scale()'s gain-scheduling hack, shared Kalman tuning
across the Smith controller's 3 filters, matplotlib imported at
module level in production code, no automated tests, and unchecked
config access.

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

components/pid/TODO.md

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+# 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`).
+
+- [ ] **FSM thresholds aren't configurable.** `tc_constants.py`'s
+  `THRESH_HOLD_IDLE`, `THRESH_HOLD_HEAT`, `THRESH_IDLE_HEAT`,
+  `THRESH_IDLE_HOLD`, `THRESH_HEAT_HOLD`, `THRESH_HEAT_IDLE` are
+  hardcoded module-level constants shared by every controller
+  instance. Move them into `config.json`'s `TempCtrl` section like the
+  PID gains, so mash schedules can tune hysteresis without code edits.
+
+- [ ] **`Pid.scale()` is a gain-scheduling hack, not anti-windup.**
+  `pid.py`'s `scale(k)` multiplies every gain by `1/heatrate_soll_set`
+  (called from `temp_controller*.py`'s `process()`), but it's framed
+  as overshoot compensation, conflated with the real anti-windup logic
+  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()`.
+
+- [ ] **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.
+
+- [ ] **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.
+
+- [ ] **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
+  schema/dataclass validation layer at config-load would surface
+  errors immediately instead of mid-`__init__`.