feat: stable device indexing across hot-plug events

[malpern]

Summary

Make device indices survive unplug/replug cycles by mapping devices to persistent identifiers instead of using monotonically increasing counters.

Builds on the device identity pipeline from PR #5.

What this includes

1. Persistent device identification — Use a combination of vendor ID, product ID, device name, and optionally serial number or physical port path to create a stable device fingerprint
2. Index reservation — When a known device disconnects and reconnects, it gets the same index back
3. Config-driven ordering — If (defdevice ...) is used (feat: name-based device matching and defdevice config #6), the config order can determine stable indices
4. Graceful handling of new devices — Devices not seen before get the next available index

Why

Currently next_device_index increments on every register_device() call, including re-registrations after rediscover_devices(). This means a USB keyboard that gets bumped out and plugged back in gets a different index, breaking any (device N) rules.

Platform considerations

• Linux: Can use /dev/input/by-id/ symlinks or evdev device properties (vendor, product, uniq)
• Windows: Interception provides device handles with stable HW IDs
• macOS: IOHIDManager provides vendor/product ID and transport info

Dependencies

• PR feat: thread device identity for per-device switch conditions #5 (device identity pipeline)
• Pairs well with name-based matching (feat: name-based device matching and defdevice config #6)

[malpern]

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