Stream Lance removal results

Issue: #55

Goal

Keep LanceEngine::remove memory for completed object deletions constant while preserving fail-fast errors and the existing data-before-manifest cleanup order.

Implementation

1. Add focused tests using drop-tracked stream items for peak retention and a poll counter for fail-fast behavior.
2. Introduce a private generic drain helper that consumes each successful item immediately and returns the first stream error.
3. Route object_store::delete_stream through the helper instead of collecting a vector.
4. Run the task contract, crate tests, strict clippy, and the repository gate; obtain independent correctness review and release verification before merge.