Support filter pushdown for scanColumn

[platypii]

Summary

• extend scanColumn results with WHERE and LIMIT/OFFSET negotiation flags
• use filtered column scans for scalar aggregate fast paths
• reuse declined column scans during fallback and avoid speculative scans that cannot be reused
• defer chunk-stream creation until rows are consumed

Validation

• npm test (58 files, 1,830 tests)
• npm run lint
• git diff --check