feat(svelte): add Svelte components

[dangreen]

Adds @srcset/svelte - Svelte 5 components for responsive images.

• Picture - <picture> with a <source> per format group ordered by efficiency, and a sizes prop applied to every source
• Image - <img> with intrinsic size, lazy loading and async decoding by default, blur-up placeholder (once per mount), bindable ref to the element, pre-hydration load recovery via $effect
• Publish manifest resolves through the svelte exports condition; the legacy top-level svelte field is dropped (clean-publish cannot override it, verified with a dry run)
• Reviewed locally in two Codex passes (final pass: no findings)

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