refactor: reuse statement execution for batch commands

[chrispader]

Summary

• extract shared SQLite statement preparation and row-consumption helpers
• execute batch and imported SQL commands without materializing query result rows
• use RAII to finalize prepared statements and avoid copying batch commands
• report zero affected rows for read-only batch commands

Testing

• nr lint-cpp
• nr sqlite typecheck
• nr typecheck in example
• iOS example build on iPhone 17 Pro simulator
• nr test:harness:ios (68 tests passed)