Add debug log ring buffer with timestamps, log CLI subcommand

firmware/include/debug_log.h

@@ -0,0 +1,30 @@
+#pragma once
+#include <string>
+#include <string_view>
+#include <vector>
+#include "pico/time.h"
+#include "ring_buffer.h"
+
+struct log_entry {
+    uint32_t timestamp_us;
+    std::string message;
+};
+
+inline ring_buffer<log_entry, 32> g_debug_log;
+
+inline void dlog(std::string_view msg) {
+    g_debug_log.push(log_entry{static_cast<uint32_t>(time_us_32()), std::string(msg)});
+}
+
+inline std::vector<log_entry> dlog_drain() {
+    std::vector<log_entry> result;
+    uint16_t n = g_debug_log.used();
+    result.reserve(n);
+    for (uint16_t i = 0; i < n; i++) {
+        log_entry e;
+        g_debug_log.peek(std::span{&e, 1});
+        result.push_back(std::move(e));
+        g_debug_log.consume(1);
+    }
+    return result;
+}