docs: note grounding y_hold_min in passive-cooling capacity in pid TODO

Captures the idea of computing the HOLD-state negative floor from the
Smith controller's internal Pot model (L*(theta_ist-theta_amb)/C) instead
of the fixed -0.1 heuristic, plus the caveat that the Normal controller
has no internal plant model to draw the same estimate from.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M2ierBoxW3v7nUbDw3M2pE

components/pid/TODO.md

@@ -155,6 +155,33 @@ git history for `temp_controller.py`/`temp_controller_smith.py`).
   and re-checking the `HoldCool`/`CoolHold` thresholds in
   `temp_controller_fsm.py:14-17`), not just wiring.
 
+- [ ] **`Outer.y_hold_min` is a fixed heuristic constant, not grounded in
+  the pot's actual passive-cooling capacity.** Related to the active-cooler
+  item above. `Pot.process()` (`components/plant/pot.py:86-89`) already
+  computes `p_loss = L * M * (temp - amb)` unconditionally every tick,
+  independent of `y` - passive cooling isn't actually *driven by* the
+  negative half of `y` today, it's a physics term that happens regardless
+  of what the controller commands. `-0.1` (see the steady-state overshoot
+  fix above) was picked by estimating a typical passive loss rate by hand
+  for one plant/ambient combination; it doesn't adapt to a different pot,
+  water mass, or `theta_ist - theta_amb` delta.
+  `TempController` (Smith, `temp_controller_smith.py:12-16,22-32`) already
+  carries its own internal `Pot` model with `L`/`M`/`C` and ambient
+  temperature set (needed for the Smith prediction) - it could expose a
+  `heatrate_loss_max(theta_ist) = L * (theta_ist - theta_amb) / C * 60`
+  (K/min) from that model and use it to size `y_hold_min` (and bound
+  `pid_inner_cool`'s target) dynamically per-tick, instead of a fixed
+  config constant - the "cooling path" would always ask for exactly what
+  passive loss can actually deliver, no more and no less. Also a natural
+  stepping stone toward the active-cooler item above: same "ceiling"
+  concept, just a bigger number once real cooling power exists.
+  Caveat: `temp_controller.py` ("Normal", non-Smith) has no internal plant
+  model at all - `set_model_plant_params()`/`set_ambient_temperature()`
+  are no-ops in `TempControllerBase` and only overridden in the Smith
+  subclass - so this only works for Smith out of the box; Normal would
+  need either its own lightweight loss estimate (e.g. from observed
+  `heatrate_ist` decay while power is ~0) or keep the fixed fallback.
+
 - [ ] **`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