Zero-copy encode: pack response body in place, single shared tx_buf, drain rx loop

firmware/include/span_writer.h

@@ -2,6 +2,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <span>
 
 class span_writer {
     uint8_t *m_data;
@@ -10,6 +11,7 @@ class span_writer {
 
 public:
     span_writer(uint8_t *data, size_t capacity) : m_data(data), m_capacity(capacity) {}
+    span_writer(std::span<uint8_t> buf) : m_data(buf.data()), m_capacity(buf.size()) {}
 
     void push_back(uint8_t v) {
         if (m_size < m_capacity) m_data[m_size++] = v;
@@ -32,4 +34,12 @@ public:
     uint8_t *end() { return m_data + m_size; }
     const uint8_t *begin() const { return m_data; }
     const uint8_t *end() const { return m_data + m_size; }
+
+    span_writer subspan(size_t offset) {
+        return span_writer(m_data + offset, m_capacity - offset);
+    }
+
+    span_writer subspan(size_t offset, size_t len) {
+        return span_writer(m_data + offset, len);
+    }
 };