Add sorted index path to BenchmarkInvertedIndexDistinct

Add sortedIndexPath() benchmark method that simulates the sorted column
execution path: merge-iterates filter bitmap against contiguous doc
ranges using PeekableIntIterator.advanceIfNeeded(). This benchmarks
all three DISTINCT operator paths side-by-side:

- sortedIndexPath: O(cardinality + filteredDocs) merge iteration
- invertedIndexPath: O(cardinality) bitmap intersects() checks
- scanPath: O(filteredDocs) forward index lookups with dedup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: xiangfu0