perf(zql): cache primary index key, pkConstraint, and index lookups

[Karavil]

Note: This PR is part of an upstream contribution effort from the Goblins team (@goblinshq). Co-authored with Claude by Anthropic.

Summary

Cache frequently recomputed values in MemorySource to avoid repeated serialization and map lookups on hot paths.

Motivation

Three values are recomputed on every #fetch() call despite being constant for a given connection:

1. Primary index key: #getPrimaryIndex() calls JSON.stringify(this.#primaryIndexSort) on every invocation. With 135 pipelines, this runs thousands of times per page render for a value that never changes after construction.
2. PK constraint: primaryKeyConstraintFromFilters() recomputes the primary key constraint from filters on every fetch, even though filters are fixed at connection time.
3. Index lookup: #getOrCreateIndex() is called with the same sort+constraint parameters repeatedly for the same connection. The JSON.stringify inside it adds further serialization overhead.

Changes

• Cache #primaryIndexKey (JSON.stringify of primary index sort) in the constructor, used in #getPrimaryIndex() and index initialization
• Cache pkConstraint (primaryKeyConstraintFromFilters result) on Connection during connect(), read from cache in #fetch()
• Cache #getOrCreateIndex results per connection via indexCache Map, keyed by constraint shape + sort

Expected Performance Impact

#getPrimaryIndex() and #getOrCreateIndex() are called on every fetch. For 135 IVM pipelines processing ~200 rows each, caching eliminates thousands of redundant JSON.stringify calls and map lookups per page render. The per-connection index cache also avoids repeated string key computation for the index map.

Testing

• All 56 existing memory-source tests pass
• All 154 existing source tests pass

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Stack Order

This PR is part of a stacked series of IVM performance optimizations. Merge in order:

1. perf(zql): reduce allocations with frozen sentinel and object reuse #5609 - Allocation reduction
2. perf(zql): cache primary index key, pkConstraint, and index lookups #5610 (this PR) - Index caching
3. perf(zql): comparator fast paths + compareBounds null fix #5611 - Comparator fast paths
4. perf(zql): fuse fetch pipeline, add PK fast path, reduce allocations #5612 - Fetch pipeline fusion

Independent PRs (no conflicts): #5607 (BTree iterators), #5608 (Join optimizations)

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