[Performance] Implement IndexedDB transaction batching

[bbopen]

Summary

Each write operation currently creates a separate IndexedDB transaction. Batching multiple writes into single transactions would significantly improve throughput.

Problem

In core/gateways/indexeddb/gateway-impl.ts, each put() call creates its own transaction. IndexedDB transactions have overhead, and frequent small transactions are inefficient for bulk writes.

Proposed Solution

1. Add batch queue configuration to database options
2. Create BatchQueue at core/gateways/indexeddb/batch-queue.ts
3. Implement time-window batching:
   • Collect writes within configurable window (e.g., 10ms)
   • Execute as single transaction
4. Implement read-your-writes consistency check
5. Add batch statistics reporting
6. Create throughput benchmarks

Expected Outcomes

• Multiple writes within time window share one transaction
• Batched writes maintain atomicity
• Read-your-writes consistency maintained
• 5x throughput improvement on bulk writes (benchmark verified)