feat: Windows/macOS multi-device identity support

[malpern]

Summary

Thread actual device identity on Windows and macOS, replacing the current hardcoded device_index: 0.

Builds on the device identity pipeline from PR #5.

What this includes

Windows (Interception driver)

• The Interception driver already distinguishes devices — interception.rs has device info available
• Thread the interception device handle through to KeyEvent.device_index
• Map interception devices to stable indices

macOS (DriverKit)

• Investigate whether the Karabiner DriverKit virtual keyboard can report source device identity
• If IOHIDManager is used, it can distinguish devices — thread HID device reference to device index
• May require changes to karabiner-driverkit crate

Why

PR #5 only enables device identity on Linux. For KeyPath to use per-device features cross-platform, Windows and macOS need to thread real device identity.

Dependencies

• PR feat: thread device identity for per-device switch conditions #5 (device identity pipeline)
• Stable indexing (feat: Windows/macOS multi-device identity support #8) for consistent device identity across reconnects

[malpern]

Parent feature: malpern/KeyPath#63 (per-keyboard rule targeting)