docs: Wayland-only path — no framebuffer workarounds. Add Qt6 instrumentation.

- WAYLAND-IMPLEMENTATION-PLAN.md v2.0: document architecture decision
  that Wayland is the only supported display path. Remove all
  framebuffer fallback workarounds (offscreen QPA, redox QPA shim).
- qwaylanddisplay.cpp: add fprintf instrumentation for crash diagnosis;
  skip xkb_context_new on Redox to eliminate potential xkb crash vector.
- greeter-ui/main.cpp: remove QT_QPA_PLATFORM=redox workaround.
  The greeter must use Wayland. Accept the crash until Qt6 is fixed.
- Ruled out: relibc calloc (zeroes correctly), libwayland proxy_create
  (correct), compositor protocol (compliant). Root cause is in Qt6
  generated Wayland wrappers passing NULL to wl_proxy_add_listener.

local/recipes/system/redbear-greeter/source/ui/main.cpp

@@ -12,13 +12,6 @@ int main(int argc, char *argv[]) {
     qputenv("QT_QUICK_BACKEND", QByteArrayLiteral("software"));
     QQuickWindow::setGraphicsApi(QSGRendererInterface::Software);
 
-    // Use the Redox-native QPA plugin (libqredox.so) instead of Wayland.
-    // The Qt6 Wayland plugin crashes at null+8 during wl_registry init on Redox.
-    // The redox QPA plugin renders directly to the framebuffer via scheme:display.
-    if (qEnvironmentVariableIsEmpty("QT_QPA_PLATFORM")) {
-        qputenv("QT_QPA_PLATFORM", "redox");
-    }
-
     QGuiApplication app(argc, argv);
     QQuickStyle::setStyle(QStringLiteral("Basic"));