Add cache for chunkShapes

[brrichards]

Summary

Adds a Map<number, ChunkShape> cache to TreeShape so that repeated calls to withTopLevelLength(n) with the same small value (n < 8) return the same ChunkShape instance instead of creating a new one each time. Values >= 8 are not cached to prevent unbounded map growth.

Full heap byte size difference between objectForest vs chunkedForest tested with plain text after changes. The heap snapshots are taken after building and materializing a fresh plain text tree:

Text characters	objectForest	chunkedForest	diff
10	355,336	359,560	1.19%
100	401,224	375,664	-6.37%
1,000	943,520	706,652	-25.10%
10,000	5,334,648	2,807,992	-47.36%

[Copilot]

Pull request overview

This PR introduces a small-instance cache on TreeShape so repeated calls to withTopLevelLength(n) for small n reuse the same ChunkShape instance, reducing repeated allocations in chunked-forest shape construction.

Changes:

• Added a per-TreeShape cache for ChunkShape instances keyed by topLevelLength for values < 8.
• Added a unit test asserting caching behavior for small lengths and non-caching for lengths >= 8.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/dds/tree/src/feature-libraries/chunked-forest/uniformChunk.ts	Adds chunkShapeCache and reuses cached ChunkShape instances for small topLevelLength values.
packages/dds/tree/src/test/feature-libraries/chunked-forest/uniformChunk.spec.ts	Adds coverage to validate caching vs non-caching behavior for withTopLevelLength.

[Josmithr]

Nit: seems like we should probably assert that topLevelLength is positive (unless the ChunkShape constructor will already do that.

[brrichards]

Yeah, ChunkShape has what you described:
assert(topLevelLength > 0, 0x4c6 /* topLevelLength must be greater than 0 */);

[Josmithr]

Looks good to me! It sounded like you had done some perf analysis on this? Might be nice to include a summary of those results in the PR description for historical context.