Per-keyboard rule targeting (Karabiner-like) — device-aware predicates for switch/actions

[malpern]

Problem

Kanata can only select devices at startup (macos-dev-names-*, linux-dev-names-*). Within a single process there’s no way to target or exclude specific keyboards inside rules (layers, overrides, aliases, etc.). Users want Karabiner-style per-device behavior (e.g., external board uses layer X, built-in uses layer Y) without running multiple Kanata instances.

Goal / Outcome

Provide a device-aware predicate that can be used in switch and (over time) other action contexts, so config authors can scope rules to specific keyboards by name/hash/path.

Scope

• Expose device metadata on events (macOS DriverKit product name/hash; Linux evdev name/path; Windows optional/TBD).
• Add a device-matching predicate (device?, if-device, or switch-device) usable in switch (v1) and extendable to other constructs (e.g., conditionals, overrides).
• Keep feature opt-in; default configs unchanged.
• Docs + samples showing include/exclude patterns and fallbacks.
• Tests: parser + runtime with macOS DriverKit stub and Linux evdev mock.

Acceptance Criteria

• A single Kanata process can apply different layers/actions based on the originating keyboard.
• Clear errors/warnings when a referenced device isn’t present.
• No measurable perf regression in the hot path (device lookup O(1)).

Notes

• Upstream issue: Feature request: device name based logic on switch func jtroo/kanata#1777 (device name based logic on switch func) — "PRs welcome".
• Device name stability is limited; allow hash/path fallback.
• Windows support is optional; don’t block the MVP.