Add ISO socket support

[colemancda]

Summary

Adds isochronous channel (LE Audio) socket support, the last kernel Bluetooth protocol missing from the package:

• BluetoothSocketProtocol.iso — the protocol was absent from the enum entirely (BTPROTO_ISO, sequenced packet).
• ISOSocket — bind, client connect (non-blocking with poll and timeout), server listen/accept, seqpacket send/receive, and get/set of the stream quality of service. The client constructor applies QoS before connecting, as the kernel requires.
• ISOSocketAddress — address plus LE address type over a new sockaddr_iso interop struct.
• BluetoothSocketOption.ISOQualityOfService — the BT_ISO_QOS option modeling unicast stream parameters: CIG/CIS identifiers (defaulting to unset), sleep clock accuracy, packing, framing, and input/output parameters (SDU interval, latency, SDU size, PHY, retransmission number) with layout matching the kernel struct.

Scope note: this covers unicast (connected isochronous streams). Broadcast isochronous streams (BIG/BIS, the extended QoS variant, and the broadcast address extension) are a follow-up. Requires kernel 6.0+; on older kernels the ISO socket is gated behind an experimental feature toggled via the management interface (#55).

Test plan

• 4 new unit tests covering protocol and option identifiers, quality of service struct layout asserted against the kernel ABI (including field offsets), and byte-level QoS encoding round-trips; 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 isochronous streams require Linux 6.0+ and LE Audio capable controllers