Add element registry for semantic E2E test automation

Replace hardcoded pixel coordinates with semantic element lookups in tests.
The element registry (added to wxWidgets) tracks all wxWindow instances,
enabling tests to find buttons by label text instead of pixel positions.

Changes:
- Add element-tracker.ts with clickByLabel, findByLabel, findByType, etc.
- Migrate clipboard, dialog, timer, filedialog, logerror tests to use registry
- Update fixtures.ts to export element-tracker utilities
- Update README with element registry documentation
- Update wxwidgets submodule with registry implementation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/e2e/utils/fixtures.ts

@@ -24,3 +24,6 @@ export const test = base.extend<{
 
 export { expect } from '@playwright/test';
 export { MAIN_CANVAS, waitForApp, tryLoadApp, getCanvasBox };
+
+// Element tracking utilities for semantic element identification
+export * from './element-tracker';