Initial commit: wireframe packet capture/analysis tool

Terminal packet capture and analysis tool built to learn the C++
memory model (byte layout, alignment, endianness, std::span over
unowned buffers) via a real capture pipeline.

- Hand-rolled L2-L4 decoders (Ethernet, IPv4, IPv6 with extension
  header walking, TCP, UDP) over std::span, no struct-casting
- L7 dissector interface with DNS, HTTP, and TLS SNI implementations
- pcapng read/write for Wireshark-compatible capture files
- Bounded capture queue: drop-on-backpressure for live capture,
  blocking push for faithful file replay
- Kernel-level BPF filtering (-f) and a separate display-only search
  (-g / interactive) that doesn't touch what's captured
- Replay mode (-r) reads a saved pcapng file back through the same
  pipeline as live capture, no root or live device needed
- pcap_stats() surfaces kernel/interface drops invisible to the
  capture queue's own counter
- Three frontends sharing one CaptureSession setup path: CLI, TUI
  (FTXUI, primary), GUI (Dear ImGui + SDL3, secondary)
- 89 unit tests (doctest) plus 9 libFuzzer harnesses covering every
  hand-rolled parser; fuzzing found and fixed a real OOM in the
  pcapng reader (unbounded allocation from an untrusted length field)

1 files changed, 54 insertions, 0 deletions

diff --git a/include/wireframe/net/tcp.hpp b/include/wireframe/net/tcp.hpp
new file mode 100644
index 0000000..f691a7f
--- /dev/null
+++ b/include/wireframe/net/tcp.hpp
@@ -0,0 +1,54 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <span>
+
+#include "wireframe/byteio.hpp"
+
+namespace wireframe::net {
+
+// Lower 6 bits of the flags byte: URG ACK PSH RST SYN FIN. CWR/ECE (the
+// top 2 bits) are masked off - not needed for now.
+inline constexpr std::uint8_t kTcpFin = 0x01;
+inline constexpr std::uint8_t kTcpSyn = 0x02;
+inline constexpr std::uint8_t kTcpRst = 0x04;
+inline constexpr std::uint8_t kTcpPsh = 0x08;
+inline constexpr std::uint8_t kTcpAck = 0x10;
+inline constexpr std::uint8_t kTcpUrg = 0x20;
+
+struct TcpHeader {
+    std::uint16_t src_port;
+    std::uint16_t dst_port;
+    std::uint32_t seq;
+    std::uint32_t ack;
+    std::uint8_t data_offset;  // header length in 32-bit words
+    std::uint8_t flags;
+    std::uint16_t window;
+};
+
+struct TcpSegment {
+    TcpHeader header;
+    std::span<const unsigned char> payload;
+};
+
+inline std::optional<TcpSegment> parse_tcp(std::span<const unsigned char> bytes) {
+    if (bytes.size() < 20) return std::nullopt;
+
+    std::uint8_t data_offset = static_cast<std::uint8_t>(bytes[12] >> 4);
+    std::size_t header_len = static_cast<std::size_t>(data_offset) * 4;
+    if (header_len < 20 || bytes.size() < header_len) return std::nullopt;
+
+    TcpHeader header{};
+    header.src_port = read_be16(bytes, 0);
+    header.dst_port = read_be16(bytes, 2);
+    header.seq = read_be32(bytes, 4);
+    header.ack = read_be32(bytes, 8);
+    header.data_offset = data_offset;
+    header.flags = bytes[13] & 0x3F;
+    header.window = read_be16(bytes, 14);
+
+    return TcpSegment{header, bytes.subspan(header_len)};
+}
+
+}  // namespace wireframe::net