fix: migrate Legacy*.svelte runtime mocks to Svelte 5 runes syntax

[XIYO]

Fixes #337

Problem

Vite 8 introduced Rolldown as the bundler, which pre-bundles .svelte files in node_modules via the vite-plugin-svelte:optimize plugin. Rolldown rejects Svelte 4 export let syntax as legacy_export_invalid, causing Storybook to silently render a completely blank preview with no error output.

Affected files:

• src/runtime/LegacyTemplate.svelte — export let id triggers the error
• src/runtime/LegacyStory.svelte — export let props trigger the error

Fix

Converted both files from Svelte 4 syntax to Svelte 5 $props() runes:

LegacyTemplate.svelte

- export let id = 'default';
+ const { id = 'default', children }: Props = $props();
+ void id;

LegacyStory.svelte

- export let name: string | undefined = undefined;
- // ... other export lets
+ const { children }: Props = $props();

Why this is safe

These files are mock stubs — they are never actually rendered. Their sole purpose is IDE type support. The real work happens in the Vite pre-transform codemod which rewrites <Template> and <Story> tags into {#snippet} and defineMeta calls at the AST level, before Svelte ever sees the components.

Converting to $props() has zero behavioral impact.

Tested with

• Vite 8.0.x (Rolldown)
• Svelte 5.55.x
• @storybook/sveltekit 10.x

[changeset-bot]

🦋 Changeset detected

Latest commit: 5ce1f92

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

This PR includes changesets to release 1 package

Name	Type
@storybook/addon-svelte-csf	Patch

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