Follow-up to cloudflare#6547, which fixed the deferred startup path but missed
two additional crash vectors for the same root cause (cloudflare#6441).

startRequest()` for the case where `isolate->service == kj::none`
(async startup not yet complete). However, the crash reported in cloudflare#6441
also reproduces on the synchronous startup path, and with the same
pattern on `ActorClassImpl::whenReady()`.

The core problem: when JS code chains temporary objects like

    loader.get(name, getCode).getEntrypoint().evaluate(args)

V8 can GC the Fetcher mid-request. This destroys the
SubrequestChannelImpl, which releases its Rc<WorkerStubImpl>, which
triggers WorkerStubImpl::unlink() → WorkerService::unlink(), clearing
the LinkedIoChannels. The child worker's IoContext still holds raw
pointers (via NullDisposer) to the WorkerService as its
IoChannelFactory and LimitEnforcer, so the next I/O operation (e.g.
an RPC callback to the parent) dereferences freed memory → SIGSEGV
or SIGBUS.

This remains 100% reproducible on current main using the reproduction
from cloudflare#6441 (@cloudflare/codemode DynamicWorkerExecutor).

Two additional fixes, both in WorkerLoaderNamespace:

- SubrequestChannelImpl::startRequestImpl(): Attach
  kj::addRef(*this) to the returned WorkerInterface, keeping the
  SubrequestChannelImpl (and thus WorkerStubImpl and WorkerService)
  alive for the full request duration. This is the fix for the
  synchronous startup path that cloudflare#6547 did not address.

- ActorClassImpl::whenReady(): Replace raw `[this]` capture with
  `[self = kj::addRef(*this)]` — same pattern as the
  SubrequestChannelImpl fix from cloudflare#6547, applied to the actor class
  deferred startup path.

Requires `@cloudflare/codemode` and `wrangler`:

```json
// package.json
{ "dependencies": { "@cloudflare/codemode": "^0.3.2", "wrangler": "^4.77.0" } }
```

```jsonc
// wrangler.jsonc
{
  "name": "repro",
  "main": "src/index.ts",
  "compatibility_date": "2025-06-01",
  "compatibility_flags": ["nodejs_compat"],
  "worker_loaders": [{ "binding": "LOADER" }]
}
```

```ts
// src/index.ts
import { DynamicWorkerExecutor, resolveProvider } from '@cloudflare/codemode';
interface Env {
  LOADER: ConstructorParameters<typeof DynamicWorkerExecutor>[0]['loader'];
}
export default {
  async fetch(request: Request, env: Env) {
    const executor = new DynamicWorkerExecutor({ loader: env.LOADER, timeout: 30_000 });
    const tools = {
      get_items: async () =>
        Array.from({ length: 112 }, (_, i) => ({
          id: `item_${i}`, name: `Item ${i}`, memo: 'x'.repeat(220),
        })),
    };
    for (let i = 0; i < 6; i++) {
      const result = await executor.execute(
        `async () => { return await codemode.get_items(); }`,
        [resolveProvider({ name: 'codemode', tools })]
      );
      if (result.error) return Response.json({ round: i, error: result.error }, { status: 500 });
    }
    return Response.json({ ok: true });
  },
};
```

Then: `wrangler dev` and `curl http://localhost:8787` → segfault every time.

To test a local workerd build against this reproduction:

    MINIFLARE_WORKERD_PATH=bazel-bin/src/workerd/server/workerd wrangler dev

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>