feat: content primitives : Callout, Card, Steps, FileTree, CodeGroup + stable razor:preview

[Shewart]

Summary

Ships the six markdown-authorable content primitives fumadocs-style docs need, and stabilises the razor:preview block so nested-component previews stop rendering as literal HTML.

New primitives (auto-registered by AddShellDocs, so <Callout> / <Card> / etc. work in any .md file with zero wiring):

Component	Purpose
<Callout Variant="info|warning|danger|tip">	Coloured info box with icon + title + prose
<Card Title Description Href IconSvg>	Anchor-or-div card; auto <a> when Href is set
<CardGrid Columns="1|2|3">	Responsive grid wrapper for <Card> / <LinkCard>
<LinkCard Title Description Href>	Horizontal arrow-decorated navigation card
<Steps> / <Step Title>	CSS-counter numbered ordered list with a rail
<FileTree> / <FileTreeItem Name IsFolder Highlight Comment>	Recursive project-layout diagram
<CodeGroup SyncKey> / <CodeTab Label>	Tabbed code samples that sync across the page via a scoped CodeGroupSyncState

Preview infrastructure fixes — the tab UI in PreviewFrame was replaced with a shadcn-style stacked layout:

• Preview always sits on top, code teaser (~8.5rem) below with a gradient fade + centered View Code button; Collapse and Copy buttons appear top-right on expand.
• Both preview and code stay mounted for the lifetime of the frame — Blazor never dismounts the DynamicComponent, so component state survives, and Shiki's one-shot DOM mutation on the code element is never re-diffed. Kills the whole class of tab-switch bugs.
• PreviewSlot now carries ChildContentRaw, extracted between the opening/closing tags of a razor:preview block. A shared SlotRenderer.FromMarkup helper recursively renders that raw markup as real nested DynamicComponents.
• SlotRenderer.BuildParameters reflects on the target component type and coerces string attribute values to the declared [Parameter] type — IsFolder="true" → bool true, Columns="2" → int, Variant="info" → enum, all cached per-type via a PropertyInfo map. Fixes InvalidCastException on <FileTreeItem IsFolder="true">.
• SlotRenderer.Dedent strips common leading whitespace before feeding nested markup back through Markdig — otherwise 4-space-indented children of <Steps> / <FileTree> were treated as indented code blocks and rendered as <pre> around placeholder divs.

Docs — added card.md, steps.md, filetree.md, code-group.md to the preview app; updated meta.json sidebar order.

Tests — renamed the local Callout test double to CustomWidget in AddShellDocsTests to avoid resolution collision with the newly auto-registered production Callout.