pid: replace IDLE-as-cooling with a real COOL state

IDLE conflated two unrelated things: "controller disabled" and "needs
to cool, which this heat-only actuator fakes via zero output." Split
them apart:

- IDLE now means disabled only - the FSM forces it whenever
  enabled=False, regardless of the temperature gap, and process_pid()
  zeroes y for it same as before.
- New COOL state (mirroring HEAT) takes over the negative-diff case,
  with its own pid_cool (separate "Cool" gains, alongside Hold/Heat)
  instead of reusing pid_rate. Its output can legitimately be negative
  - it's the actuator (tasks/heater.py's actor(), already max(0, ...))
  that clamps it to 0 because *this* plant (Pot) can only heat. A
  plant with real cooling capability could one day honor it directly.
- Thresholds renamed accordingly (HoldIdle->HoldCool, HeatIdle->
  HeatCool, new CoolHold/CoolHeat); config.json/.sim/.templ and the
  hand-rolled ctrl_params in scripts/demos/pid/ and demo_sud.py
  updated with a "Cool" params section (mirrors "Heat" for now, since
  there's no real cooling actuator to tune against yet).

Also fixes a regression in demo_sud.py/the other PID demos: none of
them ever called set_enabled(True), so since enabled defaults to
False they never drove the heater at all - only caught because this
change's demo_sud.py re-run got stuck in RAMPING forever.

config.json.templ

@@ -22,13 +22,19 @@
 			"kd": 0.0,
 			"kt": 1.5
 		},
+		"Cool": {
+			"kp": 0.05,
+			"ki": 0.01,
+			"kd": 0.0,
+			"kt": 1.5
+		},
 		"Thresholds": {
-			"HoldIdle": 0.1,
 			"HoldHeat": 1.0,
-			"IdleHeat": 1.0,
-			"IdleHold": 0.1,
+			"HoldCool": 0.1,
 			"HeatHold": 1.0,
-			"HeatIdle": 1.0
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
 		}
 	},
 	"Heater": {