docs(niri): own the portable desk and the no-match defect

displays.md still described desk-2-portable as an EDID-pending skeleton
and left the PM1601 identity unnamed. record the second desk's rig,
its position math, and the law that a rig with no eligible profile runs
the previous desk's values rather than "unchanged" — the failure that
left the portable panels on 3440x1440@180 fallbacks for weeks.

omp-docs/tools/niri/index.md

@@ -18,7 +18,7 @@ Niri is a scrollable-tiling Wayland compositor. Windows are arranged in columns
     ├── autostart.kdl   # Startup applications (noctalia, handy --start-hidden)
     ├── display.kdl     # SYMLINK to the active desk-*.kdl profile (see displays)
     ├── desk-1-home.kdl      # Desk profile: home rig (EDID-keyed)
-    ├── desk-2-portable.kdl  # Desk profile: portable monitor (skeleton)
+    ├── desk-2-portable.kdl  # Desk profile: portable rig (2x PM1601 portrait)
     ├── input.kdl       # Keyboard, touchpad, mouse
     ├── keybinds.kdl    # All key bindings
     ├── layout.kdl      # Gaps, column widths, struts