Add SCO socket support

[colemancda]

Summary

Implements the SCO socket (previously an empty stub), enabling synchronous voice links with classic (BR/EDR) devices:

• SCOSocket — bind, client connect (non-blocking with poll and timeout), server listen/accept, seqpacket send/receive, and typed accessors for socket options (MTU), connection information (handle and device class), and the voice setting.
• SCOSocketAddress — public address type over the existing sockaddr_sco interop struct, with little-endian conversion matching the L2CAP convention.
• BluetoothSocketOption.Voice — implements the previously declared but unimplemented BT_VOICE option with CVSD (0x0060) and transparent (0x0003) air coding settings; transparent is required for wideband speech codecs such as mSBC. The client constructor applies the voice setting before connecting, since the kernel requires that ordering.

Test plan

• 5 new unit tests covering option level and identifiers, voice setting values and byte-level encoding round-trips, and option struct layouts; the socket address encoding test is Linux-gated (Bluetooth address family is unavailable on Darwin) and runs in CI
• Full test suite passes
• swift build succeeds with no new warnings
• Live audio links require Linux, an existing ACL connection, and a headset-class device