feda075dc4
Pure-Go RFC 791/793 checksum implementation. Mutates buffer in place — no allocations on the hot path. Used by the redirect layer to NAT-rewrite Discord packets to 127.0.0.1:listener_port before reinjecting via WinDivertSend. UDP support deferred to P2.2. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>