Rv writes user message without awaiting A.initialization, causing stdin race at CLI

### What breaks

The SDK writes the `initialize` control_request and the first user message to the CLI's stdin with no await between them. The CLI's `initialize` handler is what populates the SDK MCP server map. When the CLI starts processing the user message before it drains the `initialize` message, the first `messages.create` is sent without the SDK MCP tools. That makes `tools[]` differ between Turn 1 and Turn 2, which breaks Anthropic's prompt cache on Turn 2.

On workloads where cache read should approach 100%, we saw it drop to around 82%.

### Versions

- `@anthropic-ai/claude-agent-sdk-typescript`: 0.2.108
- Reproduced against `@anthropic-ai/claude-code` 2.1.77 and 2.1.110

### Location

In `assistant.mjs`, the `Rv` function (around byte 1,207,734 of the minified file) writes the user message to the CLI's stdin. The SDK already has an `A.initialization` promise that resolves on the CLI's `initialize` control_response. `Rv` does not wait for it.

### Proposed fix

One line inside `Rv`:

```js
async function Rv(A, Q, B, I) {
await A.initialization; // <-- add this line
// ... existing body
}
```

The promise exists. It resolves when the CLI acks the initialize. `Rv` just needs to await it before writing the user payload.

### Caveat

In our testing this one-line fix passed 9/13 back-to-back runs, while a CLI-side gate that waits for the same signal passed 10/10. So there is probably a second race we have not traced. That said, writing the user message before the CLI has acked init is a weird invariant to live with on its own, and this fix is the one-liner in the right place. Worth taking even if a residual race turns up later.

### Related issues

- #89 (closest existing discussion of MCP tool ordering breaking cache)
- #274 (related initialize-ack semantics)
- Companion issue on the CLI side: anthropics/claude-code#49753