feat(encoding): rewrite DeltaEncoding to FastLanes wire format

Replaces custom 1-buffer bit-packing with the 2-child FastLanes delta
layout (bases + deltas as VORTEX_PRIMITIVE children) matching Rust's
fastlanes.delta wire format. Adds DeltaMetadata protobuf message.

Fix ScanIterator to not auto-close the last chunk's arena on exhaustion;
callers must call close() explicitly. Update multi-chunk tests to process
each chunk inline before advancing the iterator, matching the streaming
contract where hasNext() invalidates the previous chunk's arena.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1