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/display.h

@@ -0,0 +1,31 @@
+#ifndef DISPLAY_H
+#define DISPLAY_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// Initialize the OLED display
+void display_init(void);
+
+// Clear the display
+void display_clear(void);
+
+// Show latency value in milliseconds
+void display_latency(float latency_ms);
+
+// Show "WAITING..." message
+void display_waiting(void);
+
+// Show "TIMEOUT" message
+void display_timeout(void);
+
+// Show statistics (min/max/avg)
+void display_stats(float min_ms, float max_ms, float avg_ms);
+
+// Show mode indicator
+void display_mode(const char* mode);
+
+// Update display (call periodically)
+void display_update(void);
+
+#endif // DISPLAY_H