(feat) Add PROXY protocol v2 (PPv2) header support

[zongqi-wang]

Summary

• Add --proxy SRC_ADDR[#SRC_PORT]-DST_ADDR[#DST_PORT] CLI option to prepend PROXY protocol v2 headers to DNS queries
• Supports IPv4/IPv6, UDP (per-datagram), TCP/DoT/DoH (per-connection), with address family auto-detection
• Extract Protocol enum into protocol.h to break circular dependency with new proxy.h/proxy.cpp module
• Cache the generated PPv2 header in TrafGen to avoid per-packet reconstruction
• Add comprehensive test suite (30+ tests) covering all protocol/family combinations, address encoding, port boundaries, round-trips, and error cases
• Update README with usage examples and detailed feature documentation

Test plan

• meson test passes (all 30+ proxy protocol tests)
• ninja builds cleanly with warning_level=3
• Manual test: flame --proxy 127.0.0.1#4321-127.0.0.2#53 <target> over UDP
• Manual test: flame -P tcp --proxy 127.0.0.1#4321-127.0.0.2#53 <target> over TCP
• Manual test: IPv6 variant with [::1] bracket syntax
• Verify PPv2 header with packet capture — signature, ver/cmd, family/proto, addresses, ports all byte-correct

Testing notes

UDP + IPv4 proxy

$ ./build/flame --proxy 127.0.0.1#4321-127.0.0.2#53 -l 2 -c 1 -q 1 -v 2 8.8.8.8
PROXY v2 header enabled
avg pkt: 65 bytes  (28 PPv2 IPv4 header + 37 DNS query)

TCP + IPv4 proxy

$ ./build/flame --proxy 127.0.0.1#4321-127.0.0.2#53 -P tcp -l 2 -c 1 -q 1 -v 2 8.8.8.8
PROXY v2 header enabled
tcp conn.: 1  (header sent once at connection establishment)

UDP + IPv6 proxy

$ ./build/flame -F inet6 --proxy '[::1]#4321-[::1]#53' -l 2 -c 1 -q 1 -v 2 '[2001:4860:4860::8888]'
PROXY v2 header enabled
avg pkt: 89 bytes  (52 PPv2 IPv6 header + 37 DNS query)

Wire capture verification

Captured a UDP packet sent to a local listener and verified every field:

PPv2 signature valid: True
Ver/cmd byte: 0x21 (version 2, PROXY command)
Family/proto byte: 0x12 (IPv4 + DGRAM)
Address length: 12
Src: 192.168.1.100:4321
Dst: 10.0.0.1:53
DNS payload starts at byte 28, remaining 37 bytes

Design notes

• PPv2 header is generated once at TrafGen::start() and cached in _cached_proxy_header to avoid per-packet allocation overhead.
• For TCP/DoT/DoH, the header is written on the raw TCP stream before on_connect_event() (TLS handshake). libuv guarantees FIFO write ordering, so the header precedes the ClientHello.
• The --proxy flag uses AF_UNSPEC for address family auto-detection, since the PPv2 header describes the original client's addresses, not the flamethrower transport.