feat: support aggregates in subqueries

[kevin-dp]

Summary

• Add tests for aggregates (e.g. count) in child/includes subqueries, asserting per-parent behavior
• Fix processGroupBy to preserve __correlationKey through GROUP BY, so aggregates in child queries compute per-parent rather than globally

Problem

When using aggregates like count(i.id) inside an includes subquery, the correlation key that routes child results back to the correct parent was lost after the GROUP BY operator reshaped rows. This caused:

1. Single-group aggregation (select with aggregates, no groupBy): all child rows across ALL parents aggregated into one global result
2. Multi-group aggregation: rows from different parents with the same group key merged together
3. Output extraction returned undefined for correlationKey, so results couldn't be routed to parent Collections

Approach

Unlike orderBy + limit (which required a new groupedOrderByWithFractionalIndex operator because limiting is a post-grouping operation that needs explicit per-parent partitioning), aggregates don't need a new "grouped groupBy" operator. The existing groupBy operator already handles per-parent aggregation naturally — we just need to include __correlationKey in the grouping key so that rows from different parents end up in separate groups.

Concretely:

• Pass mainSource to processGroupBy when in includes mode
• Include __correlationKey in the grouping key so rows from different parents stay in separate groups (e.g. { __singleGroup: true, __correlationKey: 1 } instead of just { __singleGroup: true })
• Restore the { [mainSource]: { __correlationKey } } structure on output rows so the existing output extraction logic can route results back to the correct parent

Test plan

• 6 new tests for aggregates in child queries (single-group as Collection/toArray, nested aggregates as Collection/toArray, reactivity on add/remove)
• All 40 existing includes tests still pass
• All 94 group-by tests still pass
• Full test suite passes (1850/1850, 2 pre-existing failures in transactions.test.ts unrelated)

Note: The committed test file includes tests that fail without the fix. The implementation fix (in group-by.ts and index.ts) is in the working tree but not yet committed.

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🦋 Changeset detected

Latest commit: ac2022e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
@tanstack/db	Patch
@tanstack/angular-db	Patch
@tanstack/electric-db-collection	Patch
@tanstack/offline-transactions	Patch
@tanstack/powersync-db-collection	Patch
@tanstack/query-db-collection	Patch
@tanstack/react-db	Patch
@tanstack/rxdb-db-collection	Patch
@tanstack/solid-db	Patch
@tanstack/svelte-db	Patch
@tanstack/trailbase-db-collection	Patch
@tanstack/vue-db	Patch

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commit: fdfe4d1

[github-actions]

Size Change: +127 B (+0.13%)

Total Size: 98.6 kB

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Size Change: 0 B

Total Size: 3.85 kB

ℹ️ View Unchanged

Filename	Size
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