fix: preview fidelity : Razor child content, prose isolation, stretch layout, theme sync

[Shewart]

Summary

Fixes the four highest-priority issues found while building the ShellUI docs on 0.1.9.

Fixed

• Prose styles leaked into previews. Every .shelldocs-prose rule now skips .not-prose subtrees through a zero-specificity :where(). PreviewFrame carries the class, and consumers can add it anywhere else.
• Nested markup in preview child content was mangled. Inside razor:preview, and in <ComponentPreview> bodies, children are parsed as Razor (MarkdownRenderer.ParseRazor → SlotRenderer.FromRazor), recursively. Wrappers around nested components survive and nothing is wrapped in <p>. Inline tags in prose keep markdown bodies.
• Theme state only followed ShellDocs' own toggle. shelldocs.js watches <html class="dark">, saves every change under shelldocs-theme and pushes it to ThemeState. ThemeToggle is now a static-host-friendly [data-theme-toggle] button, with CSS choosing the icon.
• enhancedload handlers never ran. Blazor raises the event through Blazor.addEventListener, not as a DOM event. shelldocsOnEnhancedLoad(fn) now wires the theme re-apply, TOC re-init and mobile-nav close.

Added

• razor:preview stretch and Layout="stretch" on PreviewFrame / DemoPreview / ComponentPreview, so block-level examples can fill the frame.

Behaviour change

• razor:preview component bodies are Razor, not markdown, so markdown syntax inside them now renders literally.

Testing

• dotnet build: 0 warnings. dotnet test: 274 passing; PreviewFidelityTests is new.
• Example app: every page returns 200 with no preview errors and no console errors. Nested and stretch previews were checked in the DOM. The theme follows both the toggle and an outside class flip, and the saved theme and both icons update.