feat: code variants on previews, page switch, IframePreview (0.1.13-alpha)

[Shewart]

Summary

Code variants for cross-platform libraries, first for the ShellUI Native docs: one live preview with code for several platforms (MAUI and Avalonia XAML), like shadcn/ui's library tabs. Also fixes one-line component bodies being wrapped in <p>. Bumps to 0.1.13-alpha.

Added

• Code tabs on previews. <DemoPreview>, <ComponentPreview> and <IframePreview> take <CodeTab Label="…"> children with a fence inside, or CodeTabs from .razor.
  • The Code panel shows them as tabs instead of the Razor source, and copy copies the visible tab.
  • SyncKey switches blocks together across pages and remembers the choice.
  • A block missing the chosen tab shows "Not available on {label} yet" and keeps its height.
  • Without code tabs nothing changes.
• Page switch. o.AddSyncSwitch("platform", "MAUI", "Avalonia") adds shadcn-style underlined tabs to pages whose blocks use that key: under the page header, or above the first such block. <SyncSwitch SyncKey> places one by hand.
• Caption. Caption="…" adds a muted note to the toolbar; o.PreviewCaption sets a site default.
• <IframePreview Src Height Lazy SyncKey Caption> shows another page, e.g. Avalonia on WebAssembly.
  • It loads when visible (default), on a Run button (Lazy="click") or at once (none).
  • It appends ?theme=light|dark and posts { type: "shelldocs-theme", theme } on theme change.
• XAML highlighting: xaml / axaml highlight as XML. The init template loads Shiki's xml; existing sites add 'xml' to langs.

Fixed

• <Badge>v0.3.2</Badge> in prose no longer wraps its text in <p>. As in MDX, a body on the same line as its tags renders inline; multi-line bodies keep their paragraphs.

Notes

• All of it works in a static export: server-rendered, data-* attributes, shelldocs.js, no @onclick.
• razor:preview fences can't take code tabs, since their body is Razor.

Testing

• Release build 0 warnings; 337 tests pass, 15 new: code-tab parsing, variants, the missing-tab note, page switch, caption, iframe markup and inline bodies. Each commit builds and passes on its own.
• Example site checked, both interactive and as a shelldocs build static export:
  • tab switching and sync, the page switch, the missing note keeping its height, and copy;
  • lazy and click iframe loads, ?theme=, and the theme message;
  • light and dark, at desktop and 375px widths.
• Pack dry-run: all six packages at 0.1.13-alpha.
• ShellUI docs site built against this branch: 240/242 pages unchanged; the two introduction pages lose only the badge <p>.