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>

config.json.templ

@@ -25,6 +25,14 @@
 			"ki": 0.01,
 			"kd": 0.0,
 			"kt": 1.5
+		},
+		"Thresholds": {
+			"HoldIdle": 0.1,
+			"HoldHeat": 1.0,
+			"IdleHeat": 1.0,
+			"IdleHold": 0.1,
+			"HeatHold": 1.0,
+			"HeatIdle": 1.0
 		}
 	},
 	"Model": {