Make `form.fields` array proxies iterable

Description

The experimental form() API supports array fields via form.fields.versions[0].majorVersion.as("number"), but the proxy does not implement Symbol.iterator, so it cannot be used in {#each} blocks.

Current workaround

<!-- Requires a separate data source just to drive iteration -->
{#each items as _, index (index)}
  <input {...form.fields.versions[index]?.majorVersion.as("number")} />
{/each}

This forces duplicated iteration sources, awkward ?. chaining (TS index signature returns T | undefined), and prevents passing array field proxies to components expecting iterables.

Proposed

{#each form.fields.versions as version}
  <input {...version.majorVersion.as("number")} />
{/each}

Implement Symbol.iterator (and length) on create_field_proxy for paths resolving to arrays, yielding the same proxies that indexed access already produces.

Related

• #14662 — array mutation helpers (different scope)

Start at create_field_proxy, which is the entry point named in the issue, and trace how indexed access produces array field proxies. Add coverage for iteration and length on array paths, confirming that iteration yields the same proxies as indexed access. Done means form.fields.versions works directly in an {#each} block without a separate data source.