Proposal: Dynamic request batching interface for cells

Current Durable Objects use a single `fetch(request)` handler.
Every incoming request runs in its own turn, executes separate SQLite transactions, and waits for its own durability proof before responding.

For write-heavy workloads like event ingestion, counters, and pipeline stages, handling requests one-by-one creates a bottleneck around database transactions and replication syncs. celld can innovate beyond the basic Durable Object model by supporting an optional dynamic batching handler, such as `batchFetch(requests)`.

#### Why dynamic batching
1. **Higher throughput under load.** When requests queue up for an active cell, celld can pass an array of pending requests to the cell in a single turn. The cell processes the batch, writes to SQLite once, and returns an array of responses.
2. **Lower I/O and replication overhead.** Instead of running 100 separate SQLite transactions and 100 replication syncs, a batch of 100 requests runs in one atomic transaction and proves durability once.
3. **Flexible latency and throughput trade-offs.** Callers keep using standard single `fetch(req)`. Celld aggregates requests dynamically only when concurrency exists, preserving low latency when idle while maximizing throughput under load.
4. **Better fit for pipelines.** Developers using cells as stream aggregators or write-ahead buffers get native support for bulk operations without writing custom queueing mechanisms in userland.

If a cell does not define a batch handler, it continues using standard `fetch()` with zero changes.

edit: I've pushed a POC of this here: https://github.com/denoland/celld/commit/4f3faa47425c063d79d9815f73e73118f54b4d34

Start by reviewing the existing fetch(request) handler path and the linked POC commit 4f3faa47425c063d79d9815f73e73118f54b4d34. Compare how queued requests, SQLite transactions, responses, and durability proofs are handled; done means an optional batchFetch(requests) path works while cells without it retain standard fetch() behavior.

Written by the indexing model from the issue text.