perf: more assorted improvements

[triceo]

These changes focus largely (but not exclusively) on removing indirection.
Gets us a few percent here, few percent there, no regressions.

[Copilot]

Pull request overview

This pull request implements various performance optimizations across indexing and tuple flattening components, achieving incremental improvements without introducing regressions.

Key changes include:

• Refactored ComparisonIndexer to use a boolean flag instead of Comparator instances, reducing object allocations
• Optimized BiCompositeKey with custom equals/hashCode implementations to improve hash-based collection performance
• Converted FlattenItemBag from a record to a class with primitive int counter, eliminating MutableInt wrapper overhead

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
ComparisonIndexer.java	Simplified comparison logic with boolean reverseOrder flag, refactored methods using switch expressions for cleaner code organization, and added specialized single/many indexer paths
BiCompositeKey.java	Added custom equals() and hashCode() implementations optimized for hash-based collection usage
AbstractFlattenLastNode.java	Eliminated MutableInt wrapper by converting FlattenItemBag to a class with primitive counter, refactored update/retract logic for better readability, and optimized removeExtras() with backward iteration

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[sonarqubecloud]

Quality Gate failed

Failed conditions
67.3% Coverage on New Code (required ≥ 70%)

See analysis details on SonarQube Cloud