feat(backlight): support HID++ 0x1982

[kirgene]

The MX Keys backlight is HID++ 0x1982. The wrapper for it already exists in openlogi-hidpp but nothing calls it, BacklightFeature only shows up in the feature registry. Capabilities::lighting tracks 0x8070/0x8080 and stays false for these keyboards, and diag lighting drives only those two, so there's no path to the backlight on an MX Keys S.

This wires it up: openlogi-hid::backlight for the IPC-facing types (same shape as smartshift), openlogi-hid::write::backlight with route-based get_backlight and set_backlight_enabled, and an openlogi backlight [status|off|on] command.

set_backlight_enabled reads getBacklightConfig first and writes everything except the enable bit back unchanged, so mode, effect, level and the three fade-out durations survive a toggle. The effect goes out as the 0xff "do not change" sentinel. TemporaryManual maps to Automatic on the way back down since setBacklightConfig can't write it.

I put the command at top level rather than under diag because setBacklightConfig writes to NVM, and diag.rs describes itself as diagnosis that doesn't touch persistent state. Can move it if you'd rather.

The two new HidppOperation variants are appended, so bincode variant indices don't shift and the wire_format goldens still pass.

Tested on an MX Keys S behind a Bolt receiver:

$ openlogi backlight
device: MX KEYS S (slot 1 on receiver ...)
  current: enabled=true mode=automatic (ambient-light sensor) status=on (following ambient light) level=2/8

$ openlogi backlight off
device: MX KEYS S (slot 1 on receiver ...)
  current: enabled=true mode=automatic (ambient-light sensor) status=on (following ambient light) level=2/8
  read-back: enabled=false mode=automatic (ambient-light sensor) status=off (disabled by software) level=0/8

Unit tests cover the enum mapping and the CLI parsing.

No GUI changes. The panel would need a different shape than the RGB colour picker, so that seemed like a separate PR.

[greptile-apps]

Greptile Summary

Adds persistent HID++ 0x1982 keyboard-backlight control.

• Introduces shared backlight mode, status, and state types.
• Adds route-based backlight reads and enable/disable writes with read-back.
• Adds the top-level openlogi backlight [status|off|on] command and parser tests.

Confidence Score: 3/5

The PR is not yet safe to merge because toggling a backlight in TemporaryManual mode still replaces the user's selected mode with Automatic.

The current write path always maps TemporaryManual to Automatic and sends the write; the newly added CLI note explains the mutation but does not prevent the previously reported behavior.

Files Needing Attention: crates/openlogi-hid/src/write/backlight.rs; crates/openlogi-cli/src/cmd/backlight.rs

Important Files Changed

Filename	Overview
crates/openlogi-hid/src/write/backlight.rs	Implements HID++ 0x1982 state mapping, route-based reads, persistent enable writes, and post-write read-back.
crates/openlogi-hid/src/backlight.rs	Defines serializable IPC-facing backlight mode, status, and snapshot types.
crates/openlogi-cli/src/cmd/backlight.rs	Adds device selection, state display, and persistent on/off command handling.
crates/openlogi-hid/src/write/error.rs	Appends operation identifiers for backlight reads and writes.
crates/openlogi-cli/src/lib.rs	Adds parsing coverage for default status, off, device filtering, and invalid actions.

Sequence Diagram

sequenceDiagram
  participant User
  participant CLI as openlogi backlight
  participant HID as openlogi-hid
  participant Device as HID++ 0x1982 device
  User->>CLI: status / off / on
  CLI->>HID: get_backlight(route)
  HID->>Device: getBacklightConfig + getBacklightInfo
  Device-->>HID: configuration and current state
  HID-->>CLI: BacklightState
  opt off or on
    CLI->>HID: set_backlight_enabled(route, enabled)
    HID->>Device: getBacklightConfig
    Device-->>HID: existing configuration
    HID->>Device: setBacklightConfig
    HID->>Device: getBacklightConfig + getBacklightInfo
    Device-->>HID: read-back state
    HID-->>CLI: BacklightState
  end

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_{Reviews (3): Last reviewed commit: "fix(backlight): document and surface the..." | Re-trigger Greptile}