Split accessibility backends, fix CI regressions and document architecture

CI fixes for the previous commit:
- Windows: announce one BoundsChanged on the window root when the scene
  resizes, so UIA raises LayoutInvalidated again. Widgets still do not
  report their own bounds changes.
- macOS: collapse identical pending notifications per element within one
  update, so one text selection change posts a single
  AXSelectedTextChanged. Created/Destroyed keep every occurrence, in order.
- MinGW: link the UIA test harness with -municode and uuid, which fixes
  the nightly MinGW build.

Backends:
- Split the AT-SPI backend into a private header plus connection, events,
  tree/marshalling and method files. Break the 600-line D-Bus dispatcher
  into one handler per interface.
- Split the UIA backend into a private header plus core, element provider
  and text range files.
- Share hasState/hasAction in accessibilitybackend.hpp instead of three
  per-backend copies.

CI and docs:
- Run test_atspi.py --multi-window on Linux: closing a secondary window
  while an AT-SPI client stays connected.
- Add docs/articles/uiaccessibilityarchitecture.md (layer map, files,
  invariants, per-backend layout, common tasks, test matrix) and link it
  from the articles index and the accessibility guide.
- Add a unit test for the single layout notification on scene resize.

.github/workflows/eepp-linux-build-check.yml

@@ -107,6 +107,15 @@ jobs:
           /usr/bin/python3 projects/scripts/test_atspi.py --hidden --close-primary \
           > bin/unit_tests/output/atspi-regressions.log
         cat bin/unit_tests/output/atspi-regressions.log
+    - name: Native AT-SPI Secondary Window Tests
+      # Closes a secondary window while the client stays connected; the primary must keep answering.
+      timeout-minutes: 2
+      run: |
+        mkdir -p bin/unit_tests/output
+        env -u AT_SPI_BUS_ADDRESS dbus-run-session -- projects/scripts/xvfb-run-eepp \
+          /usr/bin/python3 projects/scripts/test_atspi.py --hidden --multi-window \
+          > bin/unit_tests/output/atspi-multi-window.log
+        cat bin/unit_tests/output/atspi-multi-window.log
     - name: AT-SPI Benchmark
       timeout-minutes: 5
       run: |