Adopt VGI 2 write result modes

[rustyconover]

Updates the C++ SDK to the coordinated VGI protocol 2.0 write-results contract.

• replaces the four writable/RETURNING booleans with the required write_result_modes map
• emits an empty map for the currently read-only C++ catalog implementation
• regenerates the authoritative Arrow schemas

Coordinated with Query-farm/vgi-python#75.

Validation: ./scripts/format.sh --check; 30-core CMake build; all 23 CTest tests.