stopLevel Schmitt-trigger hysteresis + dead-zone keep-alive

Levelbased control now distinguishes startLevel (rising-edge engage,
ramp foot) from stopLevel (falling-edge disengage). _stopHystRunning
flag flips TRUE crossing startLevel up, FALSE crossing stopLevel down.
While engaged AND level inside [stopLevel, startLevel] (basin draining
through the dead band), emit a configurable keep-alive percControl
(default 1 %) so MGC keeps a single pump running for a full drain
stroke instead of oscillating at startLevel.

Hard turn-off the moment level <= stopLevel — independent of ramp
scaling. Manual-mode demand=0 now also issues explicit turnOff to
keep parity with the new MGC handleInput semantics where demand<=0
means "off".

Editor preview shades the new hysteresis band; admin endpoint
exposes runtime engaged state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/nodeClass.js

@@ -66,6 +66,7 @@ class nodeClass {
         levelbased:{
           minLevel:uiConfig.minLevel,
           startLevel:uiConfig.startLevel,
+          stopLevel: uiConfig.stopLevel,
           maxLevel:uiConfig.maxLevel,
           curveType: uiConfig.levelCurveType || uiConfig.curveType,
           logCurveFactor: uiConfig.logCurveFactor,