docs: `defaultChecked` should not be used with form remote functions

Describe the bug

<script lang="ts">
  import { FormSchema } from './ids';
  import { addId } from './ids.remote';
</script>

<form {...addId.preflight(FormSchema)}>
  <fieldset>
    {#each ['windows', 'mac', 'linux'] as os, index}
      <label>
        <!-- windows is not checked -->
        <input {...addId.fields.operatingSystem.as('radio', os)} defaultChecked={!index} />
        {os}
      </label>
    {/each}
  </fieldset>
  <fieldset>
    {#each ['windows', 'mac', 'linux'] as os, index}
      <label>
        <!-- windows is checked -->
        <input type="radio" name="os" value={os} defaultChecked={!index} />
        {os}
      </label>
    {/each}
  </fieldset>
</form>

Happens on both Firefox and Chrome.

<form method="POST" action="?/remote=14dkmo%2FaddId">
  <fieldset>
    <!-- dev tools shows that it is checked, but the UI does not reflect this -->
    <label><input name="operatingSystem" type="radio" value="windows" checked="" /> windows</label
    ><label><input name="operatingSystem" type="radio" value="mac" /> mac</label
    ><label><input name="operatingSystem" type="radio" value="linux" /> linux</label>
  </fieldset>
  <fieldset>
    <label><input type="radio" name="os" value="windows" checked="" /> windows</label
    ><label><input type="radio" name="os" value="mac" /> mac</label
    ><label><input type="radio" name="os" value="linux" /> linux</label>
  </fieldset>
</form>

Reproduction

https://github.com/hyunbinseo/svelte-kit-template/tree/mre-kit-14707

Logs

System Info

System:
  OS: Windows 11 10.0.26100
  CPU: (8) x64 Intel(R) Core(TM) Ultra 7 258V
  Memory: 3.51 GB / 31.48 GB
Binaries:
  Node: 24.10.0 - C:\Users\hyunb\AppData\Local\fnm_multishells\24580_1760341048510\node.EXE
  Yarn: 1.22.22 - C:\Users\hyunb\AppData\Local\fnm_multishells\24580_1760341048510\yarn.CMD
  npm: 11.6.1 - C:\Users\hyunb\AppData\Local\fnm_multishells\24580_1760341048510\npm.CMD
  pnpm: 10.18.2 - C:\Users\hyunb\AppData\Local\fnm_multishells\24580_1760341048510\pnpm.CMD
  Deno: 2.4.0 - C:\Users\hyunb\.deno\bin\deno.EXE
Browsers:
  Chrome: 141.0.7390.76
  Edge: Chromium (140.0.3485.54)
  Firefox: 143.0.4 - C:\Program Files\Mozilla Firefox\firefox.exe
npmPackages:
  @sveltejs/adapter-auto: ^6.1.1 => 6.1.1 
  @sveltejs/kit: ^2.46.5 => 2.46.5 
  @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1 
  svelte: ^5.39.11 => 5.39.11 
  vite: ^7.1.7 => 7.1.7

Severity

blocking an upgrade

Additional Information

No response

Run the linked reproduction at https://github.com/hyunbinseo/svelte-kit-template/tree/mre-kit-14707 and compare the two fieldsets using defaultChecked. Start by tracing how the form markup for remote functions becomes the generated HTML shown in the issue. Done means defaultChecked visibly selects the first radio in the remote-function form, as it does in the ordinary form.