pumpingStation schema: shiftArmPercent + MeasurementContainer .default doc

- control.levelbased.shiftArmPercent (default 95): output % threshold that
  arms the shift on the way up. Once armed, the up-curve % at the
  filling→draining transition becomes the held value, kept until level
  drops to shiftLevel; from there it ramps to 0 % at startLevel.
- shiftLevel description updated — it is no longer the arming trigger,
  it's the level at which the held output begins ramping down.
- MeasurementContainer.js: prominent doc block on the class plus a
  JSDoc on getFlattenedOutput documenting the `${type}.${variant}.
  ${position}.${childId}` flatten format and the implicit 'default'
  childId convention. This was the #1 footgun for new dashboard
  consumers — the comments now make the rule impossible to miss.

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

src/configs/pumpingStation.json

@@ -546,7 +546,16 @@
                 "rules": {
                     "type": "number",
                     "min": 0,
-                    "description": "Level (m) that arms the shifted ramp when crossed going up. Should be ≤ maxLevel. Ignored when enableShiftedRamp is false."
+                    "description": "Level (m) at which the held output starts ramping down during draining. Must be > startLevel and ≤ maxLevel. Ignored when enableShiftedRamp is false."
+                }
+            },
+            "shiftArmPercent": {
+                "default": 95,
+                "rules": {
+                    "type": "number",
+                    "min": 0,
+                    "max": 100,
+                    "description": "Output % threshold that arms the shift on the way up. Once armed, the output value at the moment direction flips to draining becomes the held value, and stays held until level drops to shiftLevel. Disarms when level reaches startLevel."
                 }
             }
         },