Zero-copy TX: span_writer packer, static buffers, no vector returns

firmware/include/span_writer.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+class span_writer {
+    uint8_t *m_data;
+    size_t m_capacity;
+    size_t m_size = 0;
+
+public:
+    span_writer(uint8_t *data, size_t capacity) : m_data(data), m_capacity(capacity) {}
+
+    void push_back(uint8_t v) {
+        if (m_size < m_capacity) m_data[m_size++] = v;
+    }
+
+    template <class It>
+    void insert(uint8_t *, It first, It last) {
+        while (first != last && m_size < m_capacity)
+            m_data[m_size++] = *first++;
+    }
+
+    size_t size() const { return m_size; }
+    size_t capacity() const { return m_capacity; }
+    bool full() const { return m_size >= m_capacity; }
+
+    uint8_t *data() { return m_data; }
+    const uint8_t *data() const { return m_data; }
+
+    uint8_t *begin() { return m_data; }
+    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; }
+};