Add hardware probe and single-pixel test commands

- probe: toggles LD/CLK, reads DI state at each step, tests single pixel
- pixel0/pixel1: single pixel at max brightness for testing
- Increased LD pulse timing to 20us
- Increased CLK timing to 2us
- NeoPixel brightness raised to 80

include/pixel_stomp_mux.h

@@ -21,6 +21,7 @@ public:
     void show();
 
     void dump();
+    void probe();
 
 private:
     uint8_t pin_dat;