[Feature]: Divert keyboard Backlight Up/Down keys (CID 0x00E2/0x00E3) and add display-brightness actions

### Pre-flight checklist

- [x] I searched existing issues and the Roadmap. Closest: #710 (media-key capture via the OS event tap — cannot cover these keys, see below) and #431 (broader keyboard remap coverage — doesn't mention these keys or a brightness action).

### Problem / motivation

On the MX Mechanical Mini for Mac (046d:b36d, Bluetooth-direct), the backlight up/down keys (F4/F5 on this model) are handled entirely inside the firmware: with the backlight enabled they adjust LEDs on-device; with it disabled via 0x1982 they do nothing. In neither case does any event reach the host — verified empirically — so the OS-hook remapper and the system-event tap proposed in #710 structurally cannot capture them, nor can Karabiner/hidutil.

Logi Options+ diverts exactly these two controls and ships them pre-mapped to display brightness. On the Mac variant that matters more than a convenience: F1–F3 are the Easy-Switch host keys, so **without this diversion the keyboard has no display-brightness keys at all**. Users migrating from Options+ lose them entirely.

**Hardware supports it** — `openlogi diag controls` on this keyboard:

```
cid task flags capabilities
0x00e2 0x00c1 0x043a divertable, analytics-events
0x00e3 0x00c2 0x043a divertable, analytics-events
```

Tasks 0x00C1/0x00C2 are the firmware-internal backlight functions; both controls report `divertable` — the same 0x1b04 machinery already used for the existing key slots.

### Proposed solution

1. `KeyBacklightDown`/`KeyBacklightUp` slots, diverted only when bound (unbound keys keep stock firmware behavior).
2. `BrightnessUp`/`BrightnessDown` actions: macOS posts the NX system-defined brightness keys (same family as the existing volume events); Linux presses `KEY_BRIGHTNESSUP/DOWN`; Windows has no key event for display brightness, so its injector debug-logs and skips.

Implementation attached as a PR (verified live on the device above). GUI exposure of keyboard slots is left as follow-up — these two slots are config-only, consistent with the nine existing keyboard slots.

### Alternatives considered

OS-level remapping — impossible, the keys emit no host-visible events. Leaving them inert — the current state.

### Environment

OpenLogi 0.8.1 base, macOS 26.6.2, Apple Silicon, Bluetooth-direct.

Start with the existing key-slot diversion and 0x1b04 machinery, then inspect the platform-specific event injectors for macOS, Linux, and Windows. Done means bound KeyBacklightUp/Down slots divert correctly, brightness actions emit the requested platform events or debug-log and skip on Windows, while unbound keys retain firmware behavior.

Written by the indexing model from the issue text.