Scaffold C++20 screencast project with Meson, agent workflow, and phase plan

include/screencast/network/rtp_packet.h

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+#pragma once
+
+#include <cstdint>
+#include <span>
+#include <vector>
+
+namespace sc {
+
+// Minimal RTP header (RFC 3550) without extensions.
+struct RtpHeader {
+    uint8_t version = 2;
+    bool padding = false;
+    bool extension = false;
+    uint8_t csrc_count = 0;
+    bool marker = false;
+    uint7_t payload_type = 96;  // dynamic
+    uint16_t sequence_number = 0;
+    uint32_t timestamp = 0;
+    uint32_t ssrc = 0;
+
+    bool serialize ( std::span<std::byte, 12> out ) const noexcept;
+    static std::optional<RtpHeader> parse ( std::span<const std::byte, 12> in ) noexcept;
+};
+
+struct RtpPacket {
+    RtpHeader header;
+    std::vector<std::byte> payload;
+
+    std::vector<std::byte> serialize () const;
+    static std::optional<RtpPacket> parse ( std::span<const std::byte> in ) noexcept;
+};
+
+}  // namespace sc