brightness: match monitors by DRM connector, per-monitor setProc

[kanikamaxxing]

Summary

Two related bugs that combined to make per-monitor brightness sliders unusable on multi-monitor setups where Quickshell.screens and ddcutil detect enumerate monitors in a different order.

Bug 1 — shared setProc race

setProc was a single Process declared at the Brightness Singleton root, shared by every BrightnessMonitor. When two sliders fired setBrightness near-simultaneously, the second setProc.command = … overwrote the first before startDetached() had executed, so the first monitor's brightness change never reached ddcutil.

Bug 2 — ddcEntry matching cross-wires sliders

The ddcEntry lookup tried screen.model first, then fell back to first-unused-bus order. Quickshell's screen.model can be the bare model name while ddcutil includes a manufacturer prefix (e.g. H27T6 vs SKG H27T6), so the model match often fails. When Quickshell.screens enumerates monitors in a different order than ddcutil detect, the first-unused-bus fallback then assigns each BrightnessMonitor to the wrong DDC bus — slider for screen A controls bus B and vice versa.

Changes

• Per-monitor setProc — each BrightnessMonitor now owns its own Process so concurrent slider changes don't overwrite each other's command.
• DRM connector match — ddcutil detect --brief already emits DRM connector: card0-DP-1; we now parse that into entry.drmConnector and match entry.drmConnector.endsWith(screen.name) as the primary lookup. This is the most reliable cross-reference back to a ShellScreen and avoids both the model-prefix mismatch and the screen-order fallback.
• Model match is kept as a secondary fallback, and the original first-unused-bus heuristic is kept as a last resort.

Test plan

• Dual-monitor setup (DP-1, DP-2) where Quickshell enumerates them in the opposite order to ddcutil. Each monitor's slider now controls its own monitor's brightness deterministically.
• Drag both sliders simultaneously — both monitors receive their respective brightness updates (no more last-write-wins drop).

— robin