computer: Add an experimental dynamic JavaScript backend

[mattzcarey]

Adds DynamicJavaScriptBackend at @cloudflare/computer/backends/dynamic-javascript, marked experimental. It runs code like WorkerJavaScriptBackend, and it also runs a Workspace file whose default export extends DurableObject as a long-lived app: code imports the file and calls methods on an instance, and each instance runs as a facet of the host Durable Object with its own SQLite storage.

// /workspace/counter.js
import { DurableObject } from "cloudflare:workers";
export default class Counter extends DurableObject {
  increment(by = 1) {
    const total = (this.ctx.storage.kv.get("total") ?? 0) + by;
    this.ctx.storage.kv.put("total", total);
    return total;
  }
}

// exec code: the import is ctx.facets for the app (a DurableObjectFacets)
import Counter from "./counter.js";
export default async () => {
  await Counter.get("main").increment(2);
  await Counter.clone("main", "backup"); // also Counter.abort(name, reason), Counter.delete(name)
  return Counter.get("backup").increment(0);
};

Code run by the JavaScript backend starts from nothing on every run. Cloudflare OS has a shape for state that lasts: the agent writes a Durable Object class, the host runs it as a facet, and one-off code calls it. This copies that shape. It is separate from durable replay in #238 and changes no existing backend.

• importing an app gives ctx.facets for it, one facet per instance name. The runtime doesn't let code construct a DurableObjectFacets, so it copies the shape: get, abort, delete and clone on a prototype named DurableObjectFacets. get takes no startup options, and each method call on an instance crosses the existing bridge as plain data
• the app's file and imports load through the existing module graph, so node:fs, source modules and ws:* host modules work inside its methods, under the backend's limits
• editing the app's file, or anything it imports, restarts its instances on their next call and keeps their storage
• an instance runs one call at a time, a caller that passes its deadline stops waiting while the instance keeps going, and a read-only backend refuses app calls

New workerd tests in vitest.config.dynamic-javascript.ts, plus unit tests for detecting an app. npm run check, the package build, and npm test --workspace @cloudflare/computer pass. Docs are in docs/20_dynamic_javascript.md.

Follow-up, designed but not built (the "Planned: app interfaces" section of the docs page): app interfaces over RPC, as Cloudflare OS does it.

• Computer adds a host-side backend.facets(path) whose instances are RpcTargets using the same dispatcher, bridge and queue as calls from code.
• An agents SDK host, such as the pi durable harness, serves them through an openApp callable on its Cap'n Web transport, behind a show_app tool.
• The chat client runs the app's interface in a sandboxed iframe with no network, connected to the instance over a MessagePort, so the user and the agent share one live instance.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2552a45

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@cloudflare/computer	Minor
@cloudflare/dofs	Minor
@cloudflare/computer-rpc	Minor
@cloudflare/computerd	Minor

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commit: 2552a45