Regression: layout load functions no longer reflect mutations from their children

Describe the bug

Before #14298, this pattern worked:

// +layout.server.ts

export const load = async ({ locals, url }) => {
  url.pathname; // re-run this load function when the page changes
  locals.data = [];
  return {
    data,
  };
};

// +page.server.ts

export const load = async ({ locals }) => {
  const data = await doSomeAsyncWork();
  locals.data.push(data);
};

data would be populated with the contents of all child load functions. However, now that layout load functions are eagerly serialized, this capability is lost. I don't think this is the intention of #14298 because it talks about universal load functions, while this example is for server load functions.

Note that the child load function must be async to cause the bug.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-e6chfomf?file=src%2Froutes%2F%2Bpage.server.js

Logs

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.1.0 
    @sveltejs/kit: ^2.22.0 => 2.43.5 
    @sveltejs/vite-plugin-svelte: ^6.0.0 => 6.2.1 
    svelte: ^5.25.0 => 5.39.6 
    vite: ^7.0.4 => 7.1.7

Severity

blocking an upgrade

Additional Information

No response

Reproduce the issue using the linked StackBlitz and inspect src/routes/+page.server.js, along with the +layout.server.ts and +page.server.ts pattern described in the report. Compare behavior before and after #14298, focusing on an async child server load and eager layout serialization. Done means child mutations are reflected by the parent layout load again, with regression coverage for the reported case.