test: cover filtered legacy scans with stable row ids

[lance-gatefixer]

Summary

• add end-to-end coverage for filtering a legacy dataset with stable row IDs
• assert concurrent batch readahead returns all 400 rows with 400 unique IDs

Root cause

The historical legacy filtered-scan path sliced the stable row-ID sequence by output-batch offset instead of the physical selection in ReadBatchParams, allowing filtered pages to reuse IDs. The current reader uses the physical selection correctly; this regression test protects that behavior without extending the frozen legacy format.

Validation

• cargo test -p lance test_filter_legacy_dataset_with_stable_row_ids
• cargo fmt --all
• cargo clippy --all --tests --benches -- -D warnings

Fixes #3507

[lance-gatekeeper]

✅ Gate recommendation: approve.

This focused regression test covers the reported legacy stable-row-ID scan failure without changing the frozen legacy reader. Its completeness and uniqueness assertions exercise the physical-selection contract across concurrent batch readahead.