Move agentic primitive exports to a subpath

[Jason Morse (JasonVMo)]

Platforms Impacted

• iOS
• macOS
• win32 (Office)
• windows
• android

Description of changes

• publishes the primitive component API from @fluentui-react-native/components/primitives
• removes primitive exports from the package root
• exposes createFocusVisualProps_unstable from the primitive subpath and updates internal consumers
• documents the public primitive contracts and adds exact entry-point export guards

Closes #4250

Verification

• package format, lint, build, and Jest suites
• root TypeScript project-reference build
• repository lint, publishing, changeset, and link checks

Before	After
Primitive APIs were exported from the package root without a dedicated contract boundary.	Primitive APIs are exported only from the documented ./primitives subpath.

Pull request checklist

This PR has considered (when applicable):

• Automated Tests
• Documentation and examples
• Keyboard Accessibility
• Voiceover
• Internationalization and Right-to-left Layouts