Form remote function broken if a field name contains dashes

Describe the bug

A field name containing dashes like my-field-name causes the form submission to fail.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-form-dash-bug?file=src%2Froutes%2F%2Bpage.svelte

Logs

Uncaught (in promise) Error: Invalid path field-with-dash
    at split_path (form-utils.js?v=f53a3d90:74:9)
    at set_nested_value (form-utils.js?v=f53a3d90:22:19)
    at convert_formdata (form-utils.js?v=f53a3d90:61:3)
    at convert (form.svelte.js?v=f53a3d90:90:17)
    at handle_submit (form.svelte.js?v=f53a3d90:105:17)
    at HTMLFormElement.<anonymous> (form.svelte.js?v=f53a3d90:295:36)

System Info

System:
    OS: macOS 15.5
    CPU: (8) arm64 Apple M2
    Memory: 79.02 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.12.0 - ~/.nvm/versions/node/v22.12.0/bin/node
    npm: 9.6.4 - /opt/homebrew/bin/npm
    pnpm: 10.18.3 - /opt/homebrew/bin/pnpm
    bun: 1.3.0 - ~/.bun/bin/bun
  Browsers:
    Chrome: 141.0.7390.108
    Safari: 18.5
  npmPackages:
    @sveltejs/adapter-auto: ^6.1.0 => 6.1.1
    @sveltejs/kit: ^2.43.2 => 2.47.3
    @sveltejs/vite-plugin-svelte: ^6.2.0 => 6.2.1
    svelte: ^5.39.5 => 5.41.4
    vite: ^7.1.7 => 7.1.12

Severity

annoyance

Additional Information

No response

Start with the form-utils.js stack-trace entry points, especially split_path, set_nested_value, and convert_formdata, then reproduce the failure using the linked StackBlitz example with a dashed field name. Done means form submission succeeds for names such as my-field-name without the Invalid path error.