Replace absolute stall threshold with delta-based detection

- Use exponential moving average (EMA) to track normal running current
- Detect stall when current spikes above average by STALL_DELTA_THRESHOLD (2.0A)
- Add stabilization period (500ms) after motor start to let EMA settle
- Stall confirmation requires spike to persist for STALL_CONFIRM_MS (100ms)
- EMA stops updating during stall to prevent threshold creep
- Removes dependency on absolute current threshold that varied with speed

include/motor.h

@@ -45,6 +45,11 @@ private:
     int _adcOffsetRight = 0;
     int _adcOffsetLeft = 0;
     
+    // Delta-based stall detection (rolling average tracking)
+    float _currentEMA = 0;              // Exponential moving average of current
+    unsigned long _motorStartTime = 0;  // When motor started (for stabilization period)
+    bool _emaInitialized = false;       // EMA needs seeding on first reading
+    
     // Pingpong state
     bool _pingpongActive = false;
     int _pingpongBaseSpeed = 50;
@@ -61,6 +66,7 @@ private:
     float readCurrentSense(int pin);
     void calibrateCurrentOffset();
     void checkStall();
+    void resetStallDetection();
     void updatePingpong();
     int applyRandomness(int baseValue, int randomPercent);
 };