draft: track #1242 aggregate validation branch

[snissn]

Purpose

This draft PR is a stable tracking branch for the current #1242 aggregate candidate. It is intentionally not a review-sized merge PR.

Use this branch for candidate-vs-main performance runs and integration checks so results do not point at an ambiguous local worktree.

Current Contents

• Starts from the latest local aggregate refresh commit 2e15c96da2.
• Merges bench: expose #1242 update batch-shape measurements #1384 / codex/1242-batch-shape-measurement at 4ecfcacfe1.
• Includes the aggregate stack already present in tmp/1242-aggregate-refresh-1778004669, including the TreeDB collections: coalesced flush coordinator and prepared root apply #1242 collection write-domain, semantic coalescing, read allocation, prepared output, and read-only prepare slices that had been aggregated locally.

Validation So Far

Local checks after resolving the aggregate merge:

go test ./TreeDB/collections -run 'TestCollectionUpdateBatchStatsExposeIndexRunShape|TestCollectionUpdateBufferBreakdownStatsSnapshotAndAdd|TestPR3bSemantic|TestFreezeSortRunTable|TestCollectionIndexedReadOnlyPrepare' -count=1
go test ./cmd/mongo_gateway_bench -run 'TestProfileBench|TestDeriveTreeDBPhaseMetrics|TestMongoGateway' -count=1
go test ./TreeDB/collections -run '^$' -bench '^BenchmarkCollectionUpdateBatchDirectBufferedTemplateV1NewShape/batch_80$' -benchmem -benchtime=1x -count=1

Notes

• This PR should remain draft.
• Benchmark findings should cite this PR number and commit SHA.
• If another aggregate refresh supersedes this branch, update this PR description or close it in favor of the newer tracking PR.