fix(index): release IVF late-search barrier for skipped deltas

[lance-gatefixer]

Summary

• make every IVF delta participate in the late-search barrier before taking the no-more-probes shortcut
• add a regression test covering one delta that skips late search while a sibling continues probing

Root cause

The shared delta countdown was decremented only after the no-more-probes early return. When heterogeneous IVF segments took different paths, the skipped delta never released the barrier and its sibling waited forever.

Validation

• cargo test -p lance test_delta_skipping_late_search_releases_sibling -- --nocapture
• cargo test -p lance late_search -- --nocapture
• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings

Fixes #8349

[lance-gatekeeper]

✅ Gate recommendation: approve.

Every delta now joins the existing completion barrier before any no-work exit, restoring the invariant that all initial searches finish before siblings begin late probing. The focused fix preserves current query behavior and adds regression coverage for the mixed-branch case.