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.

docs/articles/uiaccessibility.md

@@ -13,6 +13,8 @@ The native backends are:
 Other platforms currently use the null backend. Accessibility does not change how widgets are
 rendered and does not require the application to redraw continuously.
 
+To work on the implementation itself, see [UI accessibility architecture](uiaccessibilityarchitecture.md).
+
 ## Enabling accessibility
 
 Native accessibility is enabled by default on supported platforms. Applications using