hysteria2: Fix obfs silently disabling socket buffer sizing

[realyxl]

SalamanderPacketConn and GeckoPacketConn embed the net.PacketConn interface, so SetReadBuffer/SetWriteBuffer are not promoted. quic-go gives up on its first assertion:

// sys_conn_buffers.go
conn, ok := c.(interface{ SetReadBuffer(int) error })
if !ok {
    return errors.New("connection doesn't allow setting of receive buffer size. Not a *net.UDPConn?")
}

No syscall is ever issued, so with obfs configured the socket stays at net.core.rmem_default (212992, ~146 packets of depth) instead of quic-go's 8 MB and overflows in bursts. Without obfs the conn arrives as a bare *net.UDPConn and sizing works, which is why the symptom only shows with obfuscation on. Upstream hysteria forwards these methods for the same reason (extras/obfs/conn.go).

Both are re-exposed through common.Cast, which follows Upstream() so client-side wrappers such as bufio.UnbindPacketConn are resolved; this mirrors hysteria/hop.go. SyscallConn is deliberately left out: wrapConn aborts listen/dial when it errors, and the client may run over a non-UDP conn through a detour dialer.

Reproducing quic-go's assertions against a real socket:

	SetReadBuffer	OOBCapablePacketConn	SO_RCVBUF	SO_SNDBUF
*net.UDPConn (reference)	true	true	786896 → 1048576	9216 → 1048576
salamander / gecko, before	false	false	786896 → 786896	9216 → 9216
salamander / gecko, after	true	false	786896 → 1048576	9216 → 1048576

OOBCapablePacketConn stays false, so quic-go keeps using ReadFrom/WriteTo and never bypasses deobfuscation. Over a non-UDP conn both return os.ErrInvalid, which wrapConn discards.

Builds and vets clean on linux, android, windows, darwin and freebsd; gofumpt, gofmt -s and gci report no changes.

The salamander.go half applies unchanged to main if you want it on the 1.13.x line.