Refresh design-backlog docs after Pot's gain/theta cleanup

Drop the now-stale "config.json.templ/.sim still carry dead
Model.kn/Plant.kn keys" note (they were deleted), note that Pot now
always starts at theta_amb (no separate initial-temp param) and that
dropping the gain (efficiency) factor entirely is a partial realism
regression, not just a fix. Point the config-validation item at the
gain key that's still dead in config.json.sim. Also update demo_sud.py
to match the Pot params Pot.__init__ actually reads.

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

components/pid/TODO.md

@@ -52,15 +52,16 @@ git history for `temp_controller.py`/`temp_controller_smith.py`).
   actually exposes a model to feed.
 
 - [ ] **Config access is unchecked `dict[key]` everywhere.**
-  `params['Hold']`, `model_params['gain']`, etc., throughout, with no
-  schema validation at load time. We already hit this bug class once
+  `params['Hold']`, `params['Td']`, 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__` — 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`).
+  the dead `TempCtrl.Kalman` / `Model.kn` / `Plant.kn` keys that used
+  to sit unused in `config.json.templ`/`.sim` (since deleted) and the
+  `Model.gain`/`Plant.gain` keys still doing the same in
+  `config.json.sim` right now — `Pot` dropped `gain` entirely, see
+  `components/plant/TODO.md`.
 
 - [ ] **`kalman.py` is now dead code in production.** Neither
   `temp_controller.py` nor `temp_controller_smith.py` uses `Kalman`