Version mapping: SDK ↔ protocol compatibility matrix

Goal: Document which SDK versions implement which protocol versions.\n\nScope\n- Add a simple matrix (e.g., SDK v2.2.x → RDCP v1.0)\n- Include policy for changes (SDK major/minor vs protocol changes)\n\nAcceptance Criteria\n- version-mapping.md committed at repo root\n- README links to mapping\n- Updated when versions change