Update remaining Svelte 5 migration across processor, demo, and docs

[TorstenDittmann]

Complete the remaining Svelte 5 migration work by removing legacy syntax and deprecated APIs from generated output, fixtures, app routes, and documentation examples.

• update transformer output to emit <script module> for frontmatter exports instead of context="module"
• align processor and transformer tests with the new output format
• migrate test Svelte modules from context="module" to <script module>
• replace legacy <slot /> usage in test components with snippet rendering via children and {@render children?.()}
• migrate demo layouts from $app/stores to $app/state
• replace $page-store access with page state access in demo route layouts
• modernize docs examples to Svelte 5 syntax:
  • <script module>
  • $props() calls
  • snippet-based children rendering instead of slots
• make processor fixture comparisons newline-tolerant using trimEnd() to avoid trailing newline mismatches across environments

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