Make FSM hysteresis thresholds configurable

THRESH_HOLD_IDLE/HOLD_HEAT/IDLE_HEAT/IDLE_HOLD/HEAT_HOLD/HEAT_IDLE in
tc_constants.py were hardcoded module-level constants shared by every
controller instance, so tuning the IDLE/HOLD/HEAT hysteresis required
a code change.

Replace them with DEFAULT_THRESHOLDS plus an optional
TempCtrl.Thresholds config section, merged at construction time in
TempControllerBase.__init__ so configs that omit the section keep
behaving exactly as before. Documented the new section in
config.json.templ; left config.json.sim without it to exercise the
default-fallback path.

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

components/pid/TODO.md

@@ -5,12 +5,11 @@ Findings from a design review, not yet acted on. See git history on
 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.
+- [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.
 
 - [ ] **`Pid.scale()` is a gain-scheduling hack, not anti-windup.**
   `pid.py`'s `scale(k)` multiplies every gain by `1/heatrate_soll_set`