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/log.md

@@ -22,6 +22,22 @@ not restated here — entry, then pointer. That is what the 2026-10-21 audit pas
 newest twelve entries: the facts stayed, the duplicates went, `git log --follow` keeps the
 long form.
 
+## 2026-10-30
+
+### A desk with no matching profile is silently misconfigured
+
+`desk-2-portable.kdl` stopped being a skeleton: the two PM1601 16" panels are
+identity-keyed (`PNP(XYM) PM1601 0000000000000`, `… demoset-1`), the laptop sits
+centred against their 1920-tall band, and `display.kdl` points at it. Law: a rig
+with no eligible profile does not run "unchanged" — it runs whatever the *last*
+profile said. Here that was the home profile, whose Sceptre identity block never
+matched and whose connectors still carried `3440x1440@180.000`, so niri fell back
+to preferred and every panel in force had a mode written for another desk.
+`desk.sh match` reporting "no desk profile matches connected outputs" is the
+defect signature, and `blankpad-desk-autoswitch` exits at its initial match —
+check `is-active` after any profile edit. Page:
+[tools/niri/displays.md](tools/niri/displays.md).
+
 ## 2026-10-29
 
 ### A vendor-cloud provider must be named the way the harness names it