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/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,
     };