Add union collections and live query unions

[matthewdavi]

Based on a Discord discussion with @KyleAMathews, this PR attempts to implement a union operator to join multiple live query collections in a single useLiveQuery call, which is needed for a number of things, among them, working around react hooks not being callable inside a loop.

🎯 Changes

This adds union and unionFromLiveQuery utilities for combining collections, with fail-fast key conflict detection and ref-counted dynamic unions driven by live query results. It also adds tests for union behavior, conflicts, and ref-counted removal.

Issue: #58

Usage:

import { union, unionFromLiveQuery } from '@tanstack/db'

const unified = union(collectionA, collectionB)

const dynamicUnified = unionFromLiveQuery(
  (q) => q.from({ feed: feedsCollection }),
  ({ feed }) =>
    createCollection(
      rssFeedsCollectionOptions({
        url: feed.url,
      }),
    ),
)

Local checks:

• pnpm tsc -p packages/db/tsconfig.json --noEmit
• pnpm --filter "./packages/**" lint (warnings pre-existing in repo)
• pnpm --filter @tanstack/db-ivm build
• pnpm --filter @tanstack/db build
• pnpm --filter @tanstack/electric-db-collection build
• pnpm --filter "./packages/**" test

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2a6197e

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