refactor: Deduplicate apply_deletions and promote KeyExistenceFilter to write module

[ztorchan]

Motivation

The apply_deletions function was duplicated across 4 files (delete.rs, update.rs, merge_insert.rs, and merge_insert/exec.rs) with nearly identical logic. Additionally, KeyExistenceFilter and related types were nested deep inside merge_insert/inserted_rows.rs, despite being used by transaction.rs at a higher abstraction level.

Changes

1. Extract apply_deletions into a shared public function in write.rs

   • Added pub async fn apply_deletions(dataset, removed_row_addrs) to write.rs as the single canonical implementation.
   • Removed the 4 duplicate implementations from delete.rs, update.rs, merge_insert.rs, and merge_insert/exec.rs.
   • Updated all call sites to use the shared function.

2. Promote KeyExistenceFilter to write/key_existence_filter.rs

   • Moved inserted_rows.rs from merge_insert/ to write/ as key_existence_filter.rs, making it a direct submodule of write.
   • Deleted the original merge_insert/inserted_rows.rs file.
   • Updated all import paths in merge_insert.rs, merge_insert/exec/write.rs, and transaction.rs to reference the new location.

3. Cleanup

   • Removed unused imports (BTreeMap, StreamExt, RoaringTreemap, Fragment, etc.) from files that no longer need them after the refactor.

Impact

• No functional changes. All existing tests pass (delete, update, merge_insert, key_existence_filter, transaction).
• Reduces code duplication (~50 lines × 4 copies → 1 shared function).
• Simplifies the import path for KeyExistenceFilter (e.g., write::key_existence_filter::KeyExistenceFilter instead of write::merge_insert::inserted_rows::KeyExistenceFilter).

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	0.00%	0 Missing and 1 partial ⚠️
rust/lance/src/dataset/write/update.rs	50.00%	0 Missing and 1 partial ⚠️

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