Document New Architecture adoption path for Fluent 2 in CONTRIBUTING.md

[KlementMultiverse]

Problem

The repository has open architectural discussions (#3971, #3968) about New Architecture support and cross-platform unification, but CONTRIBUTING.md lacks a dedicated architecture decision section. New contributors and maintainers cannot quickly understand:

1. Whether Fluent 2 components should target React Native New Architecture or legacy architecture
2. How platform-specific code (Windows, iOS, Android, macOS) should be organized
3. What the unification strategy is between @fluentui/react-native and platform-specific implementations
4. When and how to deprecate Old Architecture patterns

This creates friction during code review—PRs get feedback about architectural alignment without a documented standard.

Why this matters

• For contributors: Unclear whether to build New Arch-compatible vs. bridged components
• For maintainers: More review cycles needed to enforce undocumented patterns
• For the project: Risk of inconsistent implementations across platform teams
• For adopters: No clear guidance on which architecture track to adopt

What good looks like

CONTRIBUTING.md should have an "Architecture Decisions" section that includes:

1. Fluent 2 adoption timeline — Current target (New Arch, legacy, or both)
2. Platform matrix — Which platforms require New Arch, which still use bridging
3. Component migration path — How existing components transition from Old to New Architecture
4. Code organization rules — Where platform-specific code lives (e.g., packages/components/<ComponentName>/src/native/<platform>/)
5. Reference implementation — Link to a component that exemplifies the approved pattern

For comparison, see how React Native Windows CONTRIBUTING.md documents its New Arch adoption path.

Proposed section (example)

## Architecture Decisions

### New Architecture Support

FluentUI React Native is adopting React Native New Architecture. See [#3971](#3971) for tracking.

- **Current status:** [Alpha / In progress / Blocked by...]
- **Affected platforms:** [Windows, iOS, Android, macOS]
- **Migration timeline:** [Estimate or link to roadmap]

### Fluent 2 Unification

Components should implement Fluent 2 design tokens and be cross-platform compatible.
- Platform-specific overrides go in `src/native/<platform>/`
- Bridged code must be marked with `// @deprecated` comments linking to the New Arch alternative
- See [`packages/components/Button`](link) for a reference implementation.

Next steps

This unblocks #3971 and #3968 by creating a source of truth for architectural decisions.

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Contributed by Klement Gunndu