Automated per-framework test apps + durable-key contract tests

[asklar]

Goal

Add a dedicated test app + automated integration test per UI framework that asserts the durable element-key contract introduced in #26:

1. Present — every element has a non-empty \key\
2. Unique — no duplicate keys within a dump
3. Stable — two consecutive dumps of the same (unchanged) UI produce identical \key -> (framework,type,className,path)\ maps
4. Queryable — --query \ resolves the element

Each test must include a readiness wait (poll until the framework's subtree is present) to avoid the startup race (#27).

Coverage (this wave)

• WPF — new sample app + fixture
• WinUI3 — new sample app + fixture
• Avalonia — wire existing \ ests/avalonia_test_app\ into a fixture (note: plugins load only from %USERPROFILE%.lvt\plugins, deploy in test setup)
• Chromium — fix Chromium extension fails to load: manifest references missing icon files #29 (extension icons) + DOM/key test + document manual E2E
• ComCtl — self-contained Win32 window hosting SysListView32/SysTreeView32 + fixture

WinForms is already covered by #23.

Why

Only Win32 (Notepad) was truly exercised before; the Avalonia app was orphaned; WPF/WinUI3/live-Chromium had no tests. Manual validation during #26 confirmed keys work across all 7 frameworks, but automated tests are needed to keep them correct.

[asklar]

All framework test apps + durable-key integration tests merged (#31 Chromium, #34 ComCtl, #32 WPF, #35 WinUI3, #33 Avalonia), plus CI-race hardening (#36).