Expose two-way editor focus

[yvos]

Summary

• Add an optional two-way isFocused binding to NativeTextViewWrapper.
• Fulfill host focus requests immediately or after window attachment.
• Report actual first-responder transitions without disturbing another control.
• Preserve AppKit-owned focus when the binding is omitted.

This is the focus half of #173.

Validation

• FocusBindingTests: 7 tests passed.
• Full suite: 477 tests in 69 suites passed.
• Demo call site includes a programmatic focus button.