`select` elements change to first option on submit

Describe the bug

If you have a select element inside a form and submit, it changes to the first available option.

Reproduction

https://stackblitz.com/edit/form-select-resetting?file=src%2Froutes%2F%2Bpage.svelte

• Start the dev server.
• Change the select value.
• Click "Submit"

Logs

N/A

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (28) x64 Intel(R) Core(TM) i7-14700K
    Memory: 39.19 GB / 63.70 GB
  Binaries:
    Node: 24.16.0 - C:\Program Files\nodejs\node.EXE
    npm: 11.7.0 - ~\AppData\Roaming\npm\npm.CMD
    bun: 1.3.14 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (149.0.4022.69)
    Internet Explorer: 11.0.26100.8115
  npmPackages:
    @sveltejs/adapter-node: 5.5.5 => 5.5.5 
    @sveltejs/kit: 2.66.0 => 2.66.0 
    @sveltejs/vite-plugin-svelte: 7.1.2 => 7.1.2 
    svelte: 5.56.3 => 5.56.3 
    vite: 8.0.16 => 8.0.16

Severity

serious, but I can work around it

Additional Information

This happens regardless of the type of value used. So, this is similar to #15835, but not exactly the same. I intentionally used strings here in the linked reproduction to show that it happens even with strings.

Start with the linked StackBlitz reproduction and follow the listed steps: run the dev server, change the select value, and submit the form. Trace the form submission and select handling in the reproduction and relevant SvelteKit code; done means the selected value remains unchanged after submission, with a regression test covering the behavior.