fix: render input default values in SSR

[mturac]

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Fixes #18446.

This makes SSR serialize input defaultValue/defaultChecked as value/checked, matching the initial DOM state users get after hydration. Explicit value/checked still wins when it is non-nullish, including checked={false}.

Test plan:

• pnpm test runtime-runes -t input-default-value-ssr
• pnpm test runtime-runes -t "form-default-value"
• pnpm test
• pnpm lint
• pnpm check

I also temporarily reverted the implementation while keeping the new regression sample; pnpm test runtime-runes -t input-default-value-ssr failed in both SSR variants, then passed again after restoring the fix.

[changeset-bot]

🦋 Changeset detected

Latest commit: e00a589

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[mturac]

Closing this to avoid keeping a duplicate fix open for the same issue. #18452 is already open for #18446, so I will leave the review path there rather than split maintainer attention across two PRs.