feat(hy2): support salamander obfuscation

[darkworon]

Summary

• add Hysteria2 Salamander obfuscation support for obfs=salamander&obfs-password=...
• wrap QUIC packets with the Hysteria v2 [8-byte salt][payload] format using BLAKE2b-256 over PSK+salt
• validate obfuscation settings, accept plain, normalize method casing, and preserve URI export
• add focused tests for round-trip obfuscation, short PSK rejection, oversized packet handling, and generic PacketConn capabilities

Context

This covers the same feature area as #57, with a couple of hardening changes from testing against sing-box/Hysteria2 servers:

• avoid exposing UDP-specific buffer/syscall methods from the generic obfs wrapper
• validate the PSK before starting the QUIC handshake so invalid configuration fails fast instead of timing out
• normalize the obfs method case before validation/dialing

Test

• go test ./protocol/hysteria2/client ./dialer/hysteria2