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

firmware/include/msgpack.h

@@ -236,6 +236,12 @@ public:
     return *this;
   }
 
+  pack_result pack_uint32_fixed(uint32_t n) {
+    m_buf.push_back(format::UINT32);
+    push_big_endian(n);
+    return *this;
+  }
+
   pack_result pack_float(float n) {
     m_buf.push_back(format::FLOAT32);
     push_big_endian(n);
@@ -322,6 +328,19 @@ public:
     return *this;
   }
 
+  pack_result pack_ext16_header(char type, uint16_t len) {
+    m_buf.push_back(format::EXT16);
+    push_big_endian(len);
+    m_buf.push_back(static_cast<uint8_t>(type));
+    return *this;
+  }
+
+  pack_result pack_bin16_header(uint16_t len) {
+    m_buf.push_back(format::BIN16);
+    push_big_endian(len);
+    return *this;
+  }
+
   template <class Range>
   pack_result pack_ext(char type, const Range &r) {
     auto sz = static_cast<size_t>(std::distance(std::begin(r), std::end(r)));