fix: compute row offsets for unsorted addresses

[lance-gatefixer]

Summary

• sort row-address positions before advancing per-fragment deletion iterators
• scatter computed offsets back to the original request order
• add a regression test for descending offsets with intervening deletions

Root cause

AddRowOffsetExec reused a forward-only deletion iterator for addresses in input order. When a later address had a lower offset in the same fragment, the iterator retained the deletion count from the higher address and produced an offset that was too small.

Validation

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

Fixes #8250