Add debug logging to overflow/error paths, fix pipeline=2 drop

firmware/include/ring_buffer.h

@@ -13,15 +13,17 @@ struct ring_buffer {
     uint16_t free() const { return N - used(); }
     bool empty() const { return head == tail; }
 
-    void push(std::span<const T> src) {
-        if (src.size() > free()) return;
+    bool push(std::span<const T> src) {
+        if (src.size() > free()) return false;
         for (auto& v : src)
             data[(tail++) % N] = v;
+        return true;
     }
 
-    void push(const T& v) {
-        if (free() == 0) return;
+    bool push(const T& v) {
+        if (free() == 0) return false;
         data[(tail++) % N] = v;
+        return true;
     }
 
     void push_overwrite(const T& v) {

firmware/include/usb_cdc.h

@@ -8,9 +8,10 @@
 struct usb_cdc {
     ring_buffer<uint8_t, 8192> tx;
 
-    void send(std::span<const uint8_t> data) {
-        tx.push(data);
+    bool send(std::span<const uint8_t> data) {
+        if (!tx.push(data)) return false;
         drain();
+        return true;
     }
 
     void send(const std::vector<uint8_t>& data) {