Add firmware skeleton for RP2040

Complete working firmware including:
- CMakeLists.txt for Pico SDK build
- SSD1306 OLED driver (128x32, I2C)
- High-resolution latency measurement using hardware timer
- Debounced button with short/long press detection
- Three modes: Single, Continuous, Stats
- USB serial debugging output

Includes 8x8 font with numbers and letters for display.

firmware/include/button.h

@@ -0,0 +1,23 @@
+#ifndef BUTTON_H
+#define BUTTON_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+typedef enum {
+    BTN_NONE,
+    BTN_SHORT_PRESS,    // < 500ms
+    BTN_LONG_PRESS      // >= 500ms
+} button_event_t;
+
+// Initialize button GPIO
+void button_init(void);
+
+// Poll button state (call in main loop)
+// Returns event type when button is released
+button_event_t button_poll(void);
+
+// Check if button is currently pressed
+bool button_is_pressed(void);
+
+#endif // BUTTON_H