fix(brightness): honor explicit backlight mapping fallback

[Nivella-Raven]

Summary

• Fall back to the configured per-output backlight_device when automatic DRM/sysfs connector discovery cannot match a backlight to an active output.
• Keep automatic connector resolution unchanged when it succeeds.
• Continue honoring none and ddcutil backend preferences during fallback selection.

Motivation

Platform backlights do not necessarily have a DRM connector in their sysfs ancestry. On a ClockworkPi uConsole, the internal display uses this topology:

Wayland output: DSI-2
Backlight: /sys/class/backlight/backlight@0
Resolved device: /sys/devices/platform/backlight@0/backlight/backlight@0
Driver: ocp8178-backlight

The output is explicitly configured:

[brightness.monitor.DSI-2]
backend = "backlight"
backlight_device = "/sys/class/backlight/backlight@0"

However, backlight enumeration currently skips the device when automatic connector resolution fails, before consulting backlight_device. This means the explicit mapping cannot recover from the missing DRM ancestry.

This change consults active outputs for a matching explicit device only after automatic discovery fails. The selected candidate remains marked as a fallback rather than an exact DRM match.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Related context: #1898, #1899, and #3160.

Testing

Commands run on current main:

just format
just configure
just build
just test
git diff --check

Results:

• Formatted with clang-format 22.1.8.
• Full test suite passed: 91/91 tests.
• The identical source change was applied to the v5.0.0-beta.9 release, built as an aarch64 release package, and installed on Arch Linux ARM.
• Verified on a ClockworkPi uConsole under Niri 26.04. Brightness adjustment works after restart.
• Startup logs confirm the fallback path:

[brightness] using explicitly configured backlight 'backlight@0' for connector DSI-2
[brightness] found backlight candidate 'backlight@0' type='raw' current=11% connector=DSI-2 match=fallback
[brightness] selected backlight 'backlight@0' type='raw' connector=DSI-2 match=fallback

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Not applicable; this change has no visual UI component.

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

No new configuration key or user-facing string is introduced. This fixes fallback handling for the existing per-monitor backlight_device option.