merge chunks during attach/detach edits

[daesunp]

Description

Follow-up PR to #27153. Adds coalesceAroundSplice, the inverse of splitFieldAtIndex: merges adjacent same-shape UniformChunks along the seams a splice could have created, so repeated mid-field attach/detach operations don't leave the field permanently fragmented.

splitFieldAtIndex (from #27153) lets attach/detach land in the middle of a multi-node UniformChunk by splitting the chunk. Without a corresponding merge, every mid-chunk edit fragments the field — repeated same-shape inserts would produce an ever-growing run of small adjacent chunks. The uniformChunkNodeCountDynamicTargetMax policy field added in #27153 explicitly anticipated this work; this PR provides the merge logic it was waiting for.

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[Copilot]

Pull request overview

This PR adds a coalescing step to the chunked-forest editing pipeline so that repeated mid-field attach/detach operations don’t leave a field permanently fragmented into many adjacent same-shape UniformChunks. It introduces coalesceAroundSplice as the conceptual inverse of splitFieldAtIndex, merging same-shape UniformChunks along splice seams while respecting the dynamic per-chunk target cap.

Changes:

• Added coalesceAroundSplice (and internal tryMergeAt) to merge adjacent same-shape UniformChunks around splice seams, capped by uniformChunkNodeCountDynamicTargetMax.
• Integrated coalescing into ChunkedForest attach/detach edit paths immediately after the underlying splice.
• Added focused unit tests for coalescing behavior (including caps, shape-equality fallback, refcount behavior, and idCompressor retention) and integration tests verifying coalescing after mid-chunk detach/attach.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/dds/tree/src/feature-libraries/chunked-forest/chunkTree.ts	Implements coalesceAroundSplice and merge helper to reduce fragmentation around edit splices.
packages/dds/tree/src/feature-libraries/chunked-forest/chunkedForest.ts	Calls coalesceAroundSplice after attach and detach splices to keep fields from fragmenting.
packages/dds/tree/src/test/feature-libraries/chunked-forest/chunkTree.spec.ts	Adds a comprehensive test suite for coalesceAroundSplice behavior and edge cases.
packages/dds/tree/src/test/feature-libraries/chunked-forest/chunkedForest.spec.ts	Adds integration tests ensuring coalescing occurs after mid-chunk detach/attach edits.