perf(index): add exact null-row bitmap to zone map and bloom filter

[westonpace]

The queries X IS NULL and X IS NOT NULL are very common queries. Because validity is scattered throughout a file it is not actually that easy to answer this query without scanning the column. This means very wide columns can be very slow. Very wide columns also tend to be columns that are not good candidates for btree (too much duplicated data) or bitmap (too much cardinality).

On the flip side, a validity bitmap is pretty small, and maintaining one for each column is probably affordable, even for the lightweight zone map and bloom filter indexes. It would be nice to have consistent IS NULL speedup when a column is indexed, regardless of the index type.

---

Captures a RowAddrTreeMap of every null row address during index training. IS NULL queries can now be answered in O(1) by returning SearchResult::exact(null_rows) instead of scanning all zones, which also eliminates the downstream recheck step.

Backward-compatible: older indexes without the null_bitmap.lance file load with null_rows = None and fall back to the previous zone-scan path.

[codecov]

Codecov Report

❌ Patch coverage is 91.13300% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/bloomfilter.rs	88.88%	2 Missing and 7 partials ⚠️
rust/lance-index/src/scalar/zonemap.rs	91.75%	2 Missing and 6 partials ⚠️
rust/lance-index/src/scalar/zoned.rs	96.00%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!