fix: avoid fts corruption when a fragment is deleted

[westonpace]

This is basically the same problem encountered in #5929

If an entire fragment is deleted then, after an FTS update, the rows in that deleted fragment will still be indexed but shouldn't be. This fix uses the old_data_filter to properly prune the old data during the update.

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This PR was created with the help of Claude Opus 4.6. I take responsibility for all code.

[github-actions]

PR Review

Clean and well-structured fix. The approach of threading old_data_filter through the inverted index merge pipeline is the right one, consistent with how other scalar indices handle this.

One minor observation

build_empty_batch hardcodes schema assumptions (index.rs:~1963-1987): The new build_empty_batchmanually constructs columns to match the posting list schema, duplicating the column layout knowledge that lives ininverted_list_schema()/build_batch(). If the schema ever changes (e.g., a new column is added), this function would silently produce a mismatched batch. Consider whether you could instead reuse build_batchwith an empty compressed array, or at least add a debug assertion that the constructed batch schema matchesinverted_list_schema(self.has_positions())`.

Looks good

• The is_old tagging on PartitionSource is clean and correctly distinguishes old vs new partitions.
• The old_doc_id_map approach in the merger correctly filters docs and remaps posting list entries in a single pass.
• The single-partition shortcut bypass (len() <= 1 && old_data_filter.is_none()) is correct — can't just copy files when filtering is needed.
• Filtering out empty postings before creating PostingIterator is a good defensive measure.
• The regression test is thorough: covers merge_insert → compact → optimize → search, and validates both stale data removal and new data correctness.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	75.75%	4 Missing and 4 partials ⚠️
rust/lance-index/src/scalar/inverted/builder.rs	69.23%	0 Missing and 4 partials ⚠️
rust/lance-index/src/scalar/inverted/merger.rs	92.00%	0 Missing and 4 partials ⚠️
rust/lance-index/src/scalar.rs	80.00%	1 Missing ⚠️
rust/lance-index/src/scalar/inverted.rs	0.00%	0 Missing and 1 partial ⚠️

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