Add IndexedDB-backed collection for TanStack DB

[sonnes]

🎯 Changes

Adds a new collection package (@tanstack/indexeddb-db-collection) that persists data to IndexedDB with cross-tab synchronization support. This enables offline-first applications with TanStack DB.

I'm a big fan of TanStack DB, and I wanted to be able to use it with IndexedDB. I got Claude to implement it using the existing local storage collection as a reference.

I'm using this in my side projects, and it has been working great.

Examples:

• https://github.com/sonnes/september/blob/main/packages/chats/db.ts
• https://github.com/sonnes/september/tree/main/lib/indexeddb (first implementation)
• https://github.com/sonnes/reader/tree/main/src/db (uses this PR's implementation)

This is my first time contributing to an open source project, so feedback is welcome.

Key Features

• Persistent Storage: Data survives page reloads and browser restarts
• Cross-Tab Sync: Changes propagate to other tabs via BroadcastChannel
• Structured Data: Objects stored directly without JSON serialization

Usage

import { createCollection } from '@tanstack/db'
import { createIndexedDB, indexedDBCollectionOptions } from '@tanstack/indexeddb-db-collection'

// Create database with all stores defined upfront
const db = await createIndexedDB({
  name: 'my-app',
  version: 1,
  stores: ['todos'],
})

// Create collection using the shared database
const todos = createCollection<Todo>(
  indexedDBCollectionOptions({
    db,
    name: 'todos',
    getKey: (todo) => todo.id,
  })
)

await todos.insert({ id: '1', text: 'Learn TanStack DB', completed: false })

Design Decisions

1. Out-of-line keys: Object store uses explicit keys (not keyPath) for flexibility
2. Separate version store: Enables efficient change detection without modifying user data
3. Array keys for versions: [storeName, key] enables range queries by store
4. BroadcastChannel over storage events: IndexedDB doesn't fire storage events
5. Microtask queuing: Async IDB operations complete before sync protocol confirmation
6. Shared database instance: Collections accept a pre-created IDBDatabase instance via createIndexedDB() to avoid multiple version upgrades when adding object stores

✅ 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: d58d2d4

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