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

dotfiles/zz-blankpad/niri/.config/niri/cfg/desk-1-home.kdl

@@ -7,8 +7,12 @@
 // requires: Lenovo Group Limited 0x40A9 Unknown
 // optional: HDMI-A-1 (PM1601, dock-dependent)
 
+// This panel's EDID tops out at 3440x1440@180 (read live from niri: no 240 Hz mode
+// exists). Ask for a mode the panel does not offer and niri does not fail — it logs
+// "could not be found, falling back to preferred" and drops to 60 Hz on every
+// reconnect. Keep 180 here unless a re-read EDID says otherwise.
 output "Sceptre Tech Inc Sceptre M34 Unknown" {
-    mode "3440x1440@239.999"
+    mode "3440x1440@180.000"
     scale 1
     transform "normal"
     position x=1581 y=-600
@@ -47,7 +51,7 @@ output "eDP-1" {
     position x=-339 y=-240
 }
 output "DP-1" {
-    mode "3440x1440@120.000"
+    mode "3440x1440@180.000"
 
     scale 1.0