Clarify architecture boundaries and public API

[turinglambdaai]

Summary

This PR performs a low-risk architecture/documentation cleanup without changing existing public bindings or moving native backend files.

What changed

• rewrote the root README around the current Glaze product model, quick start, actual features, architecture, status, and package layout
• added docs/architecture.md with current layers, dependency rules, and Public/Internal/Platform-specific boundaries
• added a focused ROADMAP.md
• documented glaze/main.rkt as the compatibility-preserving public facade; exports are unchanged
• added glaze-test/public-api-test.rkt to protect (require glaze) and basic facade behavior
• added minimal examples/tray/ and examples/events/ examples using only the public facade
• reorganized the examples index around a small learning path

Compatibility

No public API was removed or renamed. Existing focused module imports remain supported. Platform backend files were not moved.

Why

The repository already has a good dispatcher/backend shape for WebView, tray, and sys. The higher-value stabilization step is to make that boundary explicit and test the facade before considering any directory migration or larger runtime abstraction.

Validation

The existing CI matrix runs raco test glaze-test/ on Windows, macOS, and Linux, plus native WebView e2e and package builds. The new facade test is included automatically by the existing directory test command.