feat: implement golden round-trip (Rust <-> Mojo) and hybrid tool-calling structure

[MrBT-nano]

📝 Description

This PR establishes the foundational typed boundaries for the vtuber-* ecosystem in Phase 1.

Key Changes:

1. Golden Round-trip Verification: Implemented a cross-language testing suite (Rust <-> Mojo/Python) to ensure contract integrity. Verified Thai Unicode support and core metadata preservation.
2. Hybrid Tool-Calling Structure: Defined tool_call.proto supporting a hybrid execution model:
   • Centralized (Brain): For reasoning-heavy tools (e.g., WebSearch).
   • Decentralized (Performer): For performance-critical tools (e.g., Animation, Sound Effects).
3. Mojo Facade: Updated the Mojo binding layer via Python interop (ADR-004) to support complex fields and enums.

🔗 Related Issues

Fixes # (Initial infrastructure setup)

🛠️ Type of Change

• 🚀 New feature
• 📄 Documentation update
• 🧪 Testing
• ⚙️ CI/CD (Fixed formatting and .gitignore)

✅ Checklist

• Tests passed locally (Rust examples and Mojo round-trip)
• Documentation updated (STRUCTURE.tree, locales)
• Code follows project style guidelines (ADR-001 to ADR-004)
• cargo fmt and buf lint pass
• mojo format and pixi run check pass

🏷️ Metadata

• Assignee: @MrBT-nano
• Labels: feat, contracts, multilingual

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