MB-69881: Re-architect vector search

[CascadingRadium]

• Use a bitset to track eligible documents instead of a slice of N uint64s, reducing memory usage from 8N bytes to N/8 bytes per segment (up to 64× reduction) and improving cache locality.
• Pass an iterator over eligible documents that iterates the bitset directly, allowing direct translation into a bitset of eligible vector IDs in the storage layer and eliminating the need for a separate slice intermediary.
• Fix garbage creation in the UnadornedPostingsIterator, which previously allocated a temporary struct per Next() call to wrap a doc number and satisfy the Postings interface; the iterator now returns a single reusable struct (one-time allocation) consistent with the working of the PostingsIterator in the storage-layer.
• Avoid unnecessary BytesRead statistics computation when executing searches in no-scoring mode, removing redundant work as a micro-optimization.

[Copilot]

Pull request overview

This PR re-architects vector search to improve memory efficiency and reduce garbage collection pressure. The changes replace slice-based eligible document tracking with bitsets, achieving up to 64× memory reduction per segment, and optimize the iterator pattern to eliminate per-call allocations in the unadorned postings iterator.

Key changes:

• Replaced slice-based eligible document tracking ([]uint64) with bitsets, reducing memory from 8N bytes to N/8 bytes per segment
• Introduced iterator-based API for eligible documents that directly translates to bitset iteration at the storage layer
• Fixed garbage creation in UnadornedPostingsIterator by reusing a single struct instance instead of allocating per Next() call
• Optimized bytes read tracking to skip computation in no-scoring mode

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
index/scorch/snapshot_vector_index.go	Introduces bitset-based eligible document storage and iterator API, replacing the previous slice-based approach
index/scorch/unadorned.go	Changes UnadornedPosting from uint64 to struct with pointer receivers and adds reusable struct fields to iterators to eliminate per-call allocations
index/scorch/snapshot_index_tfr.go	Adds conditional bytes read tracking via updateBytesRead flag to skip computation in no-scoring mode
index/scorch/snapshot_index.go	Initializes updateBytesRead flag based on scoring requirements
index/scorch/optimize_knn.go	Removes requiresFiltering flag and updates to use new SegmentEligibleDocuments API
index/scorch/optimize.go	Sets updateBytesRead to false for unadorned term field readers
index/scorch/snapshot_index_vr.go	Updates InterpretVectorIndex call to remove filtering parameter
index_test.go	Updates expected bytes read values to reflect the optimization that skips unnecessary computation

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