Add HIDP support

[colemancda]

Summary

Adds Human Interface Device Protocol support via the kernel's HIDP module, turning the previously empty HIDP target into a working session-management layer:

• HIDPSocket — control socket for kernel HID sessions: addConnection bridges connected L2CAP control (PSM 17) and interrupt (PSM 19) sockets into a kernel input device, passing the HID report descriptor, parser version, device identity (vendor/product/version), name, flags, and idle timeout; removeConnection destroys a session; connections() lists active sessions; connectionInformation(for:) queries a single device.
• Ioctl wrappers for all four operations (the identifiers already existed in HIDPIO but had no request types). The add request embeds a pointer to the caller's report descriptor buffer, and the connection list embeds a pointer to a caller-allocated array — both wired internally with the buffers kept alive for the duration of the call.
• C interop structures matching the kernel ABI, including the fixed 128-byte device name buffers (modeled as 32-bit words to preserve C alignment) and little-endian device addresses.
• Value types — HIDPConnectionFlag (virtual cable unplug, boot protocol mode), HIDPConnectionState, and a public HIDPConnection info struct.

Together with the classic L2CAP conveniences (#58), this enables HID hosting without a daemon; fetching the report descriptor via service discovery remains a follow-up.

Test plan

• 7 new unit tests using the syscall mocking driver: ioctl request codes asserted against exact kernel values, struct memory layouts asserted against the C ABI (168/12/148/16 bytes), faked kernel replies for add connection (including report descriptor pointer contents), connection list, and connection information, and device name encoding round-trips with truncation
• Full test suite passes
• swift build succeeds with no new warnings
• Live sessions require Linux with the kernel HIDP module and CAP_NET_ADMIN (root)