move callback_list, parse_buffer, prepend_buffer, static_vector, timer_queue into util; util links pico_stdlib for timer_queue

util/prepend_buffer.h

@@ -0,0 +1,41 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <span>
+
+// Buffer that grows in both directions from a midpoint. Payload is appended at
+// the back; each protocol layer then prepends its header in reverse order
+// (UDP, then IPv4, then Ethernet) without moving payload bytes. Lets the
+// network stack build outbound frames bottom-up with no copies or scratch.
+template <size_t N>
+class prepend_buffer {
+    uint8_t m_buf[N];
+    size_t m_start = N / 2;
+    size_t m_end = N / 2;
+
+public:
+    template <typename T>
+    T* prepend() {
+        m_start -= sizeof(T);
+        return reinterpret_cast<T*>(m_buf + m_start);
+    }
+
+    uint8_t* append(size_t len) {
+        uint8_t* p = m_buf + m_end;
+        m_end += len;
+        return p;
+    }
+
+    void append_copy(std::span<const uint8_t> data) {
+        memcpy(append(data.size()), data.data(), data.size());
+    }
+
+    uint8_t* payload_ptr() { return m_buf + m_end; }
+
+    uint8_t* data() { return m_buf + m_start; }
+    const uint8_t* data() const { return m_buf + m_start; }
+    size_t size() const { return m_end - m_start; }
+
+    std::span<const uint8_t> span() const { return {data(), size()}; }
+};