Move C++ tree into firmware/

firmware/include/usb_cdc.h

@@ -0,0 +1,31 @@
+#pragma once
+#include <cstdint>
+#include <span>
+#include <vector>
+#include "tusb.h"
+#include "ring_buffer.h"
+
+struct usb_cdc {
+    ring_buffer<512> tx;
+
+    void send(std::span<const uint8_t> data) {
+        tx.push(data);
+        drain();
+    }
+
+    void send(const std::vector<uint8_t>& data) {
+        send(std::span<const uint8_t>{data});
+    }
+
+    void drain() {
+        while (!tx.empty()) {
+            uint32_t avail = tud_cdc_write_available();
+            if (avail == 0) break;
+            auto chunk = tx.read_contiguous();
+            if (chunk.size() > avail) chunk = chunk.first(avail);
+            tud_cdc_write(chunk.data(), chunk.size());
+            tx.consume(chunk.size());
+        }
+        tud_cdc_write_flush();
+    }
+};