draft: track #1242 full aggregate validation branch

[snissn]

Purpose

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

Use this branch for candidate-vs-main performance runs and integration checks when the intended aggregate includes the no-index staged update semantics that were missing from #1386.

Relationship To #1386

This PR starts from #1386 head e44f1f6b15d56b861ef4de8ee7415af4a6604000, including the iterator-wrapper fast-path fix, then merges the missing/superseding heads below:

• TreeDB collections: stage primary-only no-index updates #1334 / 2874c941e8ca7a339b7657e186b7b0948008533e — stage primary-only no-index updates.
• caching: size GetMany copy arena by unique probes #1381 / adac971eb4e84fc1262be23a2b052959ece6c4d5 — size GetMany copy arena by unique probes.
• PR3b: capture raw semantic indexed update records #1336 current head / f55c7aa83f4cc7c2f9ccc0b3a5956c13afa7408d — include the current semantic-record staging head.

Current Contents

• Full draft: track #1242 aggregate validation branch #1386 aggregate validation branch.
• PR2b/TreeDB collections: stage primary-only no-index updates #1334 staged no-index Update / no-index UpdateBatch behavior.
• caching: size GetMany copy arena by unique probes #1381 read allocation guardrail fix.
• Current PR3b: capture raw semantic indexed update records #1336 semantic record staging head.

Local Verification

go test ./TreeDB/collections \
  -run 'TestCollectionRootDeltaStatsIteratorPreservesFastPathTraits|TestPrimaryOnlyNoIndexUpdateCounters|TestCollectionNoIndexUpdate|TestCollectionCheckpointDoesNotFlushPendingNoIndexUpdate|TestCollectionManagerCloseDrainsPendingPrimaryOnlyNoIndexUpdate' \
  -count=1 -timeout=3m

go test ./TreeDB/caching \
  -run 'TestGetMany_MemtablePointerValues_DefensiveCopyAndCapacityCapped|TestGetMany_DuplicateHitsReuseSingleProbeWithoutResultAliasing|TestRootDomainProbeStats_GetMany' \
  -count=1 -timeout=2m

go test ./TreeDB/docs -run '^$' -count=1 -timeout=2m
go test ./TreeDB/mongo_gateway -run '^$' -count=1 -timeout=2m

Notes

• This PR should remain draft.
• Benchmark findings should cite this PR number and commit SHA.
• This branch should be treated as the fuller aggregate candidate if TreeDB collections: stage primary-only no-index updates #1334 is part of the intended TreeDB collections: coalesced flush coordinator and prepared root apply #1242 stack.