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/latency.h

@@ -0,0 +1,38 @@
+#ifndef LATENCY_H
+#define LATENCY_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// Measurement result
+typedef struct {
+    bool valid;         // True if measurement succeeded
+    float latency_ms;   // Measured latency in milliseconds
+    uint64_t latency_us; // Raw latency in microseconds
+} measurement_t;
+
+// Statistics
+typedef struct {
+    float min_ms;
+    float max_ms;
+    float avg_ms;
+    uint32_t count;
+} stats_t;
+
+// Initialize latency measurement hardware
+void latency_init(void);
+
+// Perform a single latency measurement
+// Returns result struct with valid flag and latency value
+measurement_t latency_measure(void);
+
+// Get current statistics
+stats_t latency_get_stats(void);
+
+// Reset statistics
+void latency_reset_stats(void);
+
+// Add measurement to statistics
+void latency_add_to_stats(float latency_ms);
+
+#endif // LATENCY_H