3-way merge of amnezia-vpn/amneziawg-go (AWG2, I1-I5 CPS + S4 keepalive) into
sagernet/wireguard-go@506b763185 (the base sing-box uses). Keeps sagernet's
sing-box-compat additions (Send offset contract, InputPacket, conn reserved/
control) and neutralizes the encapsulating headroom (MessageEncapsulatingTransportSize=0)
so obfuscation composes cleanly. Obfuscation lives in device/ (obf*.go, magic-header.go
new files + send/receive/device/uapi grafts); conn/tun/ipc kept pure sagernet.
LIVE-VALIDATED: handshake + keepalive + traffic against a real AmneziaWG 2.0 server.
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