fix(niri): pin the ultrawide to the 180 Hz mode its EDID actually offers

desk-1-home.kdl asked the Sceptre M34 for 3440x1440@239.999, a mode the panel
has never offered. niri does not reject an unavailable mode: it logs "configured
mode ... could not be found, falling back to preferred" and drives the EDID
preferred mode - 60 Hz - on every connector event, which is why the monitor
looked like it was dying rather than misconfigured. Read live with
`niri msg outputs`, the mode list tops out at 180.

Also destroys the copies of the phantom number: displays.md carried 239.999 in
its diagram, its panel table and its copy-paste pattern block, and its rule 4
claimed an impossible mode leaves the output dark. Rule 4 now states the real
failure mode, a new section separates config / DP-link / amdgpu-driver faults by
log signature, and wlr-randr is corrected to not-installed (three pages claimed
it; `pacman -Qs wlr` matches nothing).

Applied live with `niri msg output <panel> mode <mode>` - the temporary
per-output path, so no config reload and no logged-out session - verified at
3440x1440 @ 180.000 Hz with the compositor PID unchanged.

Refs #58

omp-docs/tools/cachyos.md

@@ -95,7 +95,6 @@ CachyOS with Niri ships with a curated set of applications:
 - **wf-recorder** — Screen recording
 - **hyprpicker** — Color picker
 - **wl-clipboard** — Clipboard management
-- **wlr-randr** — Display management
 - **NetworkManager** — Network management
 - **BlueZ** — Bluetooth
 - **pavucontrol** — Audio mixer