B1.3: isStable real threshold (config-driven, replaces tautology)

The legacy stdDev < stdDev*2 was always true. New behaviour: stdDev <=
config.calibration.stabilityThreshold OR stdDev === 0. Default
threshold 0.01 in scaling-units. Schema field + editor UI added. 4
BUG-PRESERVED tests rewritten + 4 new edge tests. 101/101 pass.

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

src/calibration/calibrator.js

@@ -2,7 +2,7 @@
 
 const { stats } = require('generalFunctions');
 
-const MARGIN_FACTOR = 2;
+const DEFAULT_STABILITY_THRESHOLD = 0.01;
 
 /**
  * Calibration helper extracted from measurement/specificClass.js.
@@ -23,8 +23,9 @@ class Calibrator {
 
   /**
    * Decide whether the rolling window is stable enough to trust.
-   * Mirrors the original threshold check; with `stdDev=0` (constant input)
-   * the comparison short-circuits to true.
+   * Compares the window's stdDev against config.calibration.stabilityThreshold
+   * (absolute, in scaling-units). A constant buffer (stdDev=0) is always
+   * stable regardless of threshold.
    */
   isStable() {
     const values = this._storedValues();
@@ -32,8 +33,12 @@ class Calibrator {
       return { isStable: false, stdDev: 0 };
     }
     const stdDev = stats.stdDev(values);
-    const stableThreshold = stdDev * MARGIN_FACTOR;
-    return { isStable: stdDev < stableThreshold || stdDev === 0, stdDev };
+    const cfg = this._config();
+    const raw = cfg && cfg.calibration && cfg.calibration.stabilityThreshold;
+    const threshold = Number.isFinite(Number(raw)) && Number(raw) >= 0
+      ? Number(raw)
+      : DEFAULT_STABILITY_THRESHOLD;
+    return { isStable: stdDev === 0 || stdDev <= threshold, stdDev };
   }
 
   /**

src/nodeClass.js

@@ -33,6 +33,7 @@ class nodeClass extends BaseNodeAdapter {
       },
       smoothing: { smoothWindow: uiConfig.count, smoothMethod: uiConfig.smooth_method },
       simulation: { enabled: uiConfig.simulator },
+      calibration: { stabilityThreshold: uiConfig.stabilityThreshold },
       mode: { current: mode },
       channels,
     };