Add per-keyboard device targeting UI

[malpern]

Summary

Kanata already supports per-device rules via device-if in defcfg, and KeyPath preserves these when users write them manually in keypath.kbd. However, there's no UI for selecting which keyboard a rule applies to.

Karabiner-Elements has full UI support for this — users can target specific connected keyboards from the Devices tab. We should build equivalent UI support.

Current state

• Kanata device-if works if users edit config manually
• kanata-users.md documents that device filters are preserved
• No device selection in the rules UI (custom rules only support global or app-specific)
• VirtualHID exclusion is auto-generated but that's a safety mechanism, not user-facing

What's needed

• Device detection and selection UI
• Per-device rule targeting in the custom rules editor
• Config generation to emit device-if blocks
• Update karabiner-users.md to move per-device targeting from Karabiner's strengths to the comparison table once shipped

References

• Karabiner device docs: https://karabiner-elements.pqrs.org/docs/manual/configuration/configure-devices/
• Kanata device-if syntax: https://github.com/jtroo/kanata/blob/main/docs/config.adoc
• Current mention in Sources/KeyPathAppKit/Resources/kanata-users.md (Device Filtering section)

[malpern]

Implementation Note: Device Data Already Available

As of karabiner-driverkit v0.2.0 (shipped in kanata v1.10.0, November 2025), kanata --list returns structured device data including hash, vendor_id, product_id, and product_key for every connected keyboard.

KeyPath already parses this output in KanataConfigurationGenerator.swift (runKanataListForDeviceNames() + parseExcludedMacOSDeviceNames()) to auto-exclude VirtualHID devices. The data needed for a device picker UI is right there — we just discard everything except the VirtualHID matches today.

What's available per device from kanata --list

Field	Example	Notes
hash	0xA1B2C3D4E5F6	FNV-1a of vendor_id:product_id:product_key — stable device identifier
vendor_id	1452	USB VID (Apple = 0x05AC / 1452)
product_id	628	USB PID
product_key	Apple Internal Keyboard / Trackpad	IOKit product name

Backstory

This data path exists because of upstream work on psych3r/driverkit:

• PR feat: add arrow syntax for CLI map command #7 (malpern, Aug 2025) — proved VID/PID could be exposed from the driverkit C++ layer
• psych3r then did a full v0.2.0 redesign replacing the stdout-capture pattern with a structured fetch_devices() FFI, which kanata PR #1813 integrated

Suggested approach

1. Extend the existing kanata --list parser to return all device data (not just exclusion names)
2. Create a ConnectedDevice model with hash, vendor_id, product_id, product_key
3. Surface in a device picker UI — user selects which keyboards to remap
4. Generate macos-dev-names-include (or hash-based targeting) in the kanata config