feat: swap Prism for Shiki - VSCode-quality highlighting via WASM

[Shewart]

Summary

Replaces Prism (CDN scripts, per-language files) with Shiki (single ESM module, WASM-backed) for syntax highlighting. Same TextMate grammars VSCode ships, so razor, csharp, html, json, yaml, bash, typescript, javascript, and markdown all colour identically to how they'd look in your editor. Dual-theme wired (github-light + github-dark) so the sun/moon toggle flips code colours without a re-render.

Wraps up Phase 1 — only chore/release-0.1.0-alpha left before we cut the first NuGet publish.

What's in it

examples/ShellDocs.Preview/Components/App.razor

• Removed: 2 Prism theme CSS links + 7 per-language Prism JS scripts (prism-csharp, prism-json, prism-yaml, prism-bash, prism-markup-templating, prism-typescript, prism.min.js).
• Added: one <script type="module"> that pulls createHighlighter from esm.sh/shiki@1.24.0, preloads the 9 languages we support, and kicks off shelldocsHighlight() on ready. Assigned to window.__shiki for the runtime helpers to consume.

src/ShellDocs.Components/wwwroot/shelldocs.js

• shelldocsHighlight() and shelldocsHighlightElement(preEl) rewritten. Old flow: Prism.highlightAll() mutates child spans in place. New flow: pass source text + language into shiki.codeToHtml, get back a full <pre class="shiki">, replace the original <pre> in the DOM.
• Idempotent — a data-shiki="done" marker skips re-highlighting on subsequent shelldocsHighlight() calls (e.g., after a Blazor SPA nav or PreviewFrame tab flip).
• Unknown/unloaded languages are silently no-op'd via getLoadedLanguages().includes(lang) — no throw, no console spam.

src/ShellDocs.Components/wwwroot/shelldocs-theme.css

• Prism overrides (pre[class*="language-"], code[class*="language-"] with !important resets) removed.
• New pre.shiki chrome — normalises Shiki's margin/padding/font to our codeblock frame.
• Dual-theme CSS bridge: color: var(--shiki-light) in light mode; :root.dark selectors flip to var(--shiki-dark). Uses defaultColor: false on the Shiki call so each token span carries both --shiki-light and --shiki-dark CSS vars, and the theme class picks which one resolves. Instant theme flip with zero re-highlight.
• Bug fix — original CSS was applying background-color to every pre.shiki span, which included Shiki's <span class="line"> wrappers, so each line painted a grey background strip across the block. Restricted background-color to the outer <pre> only (and set to transparent so our chrome shows through); spans only carry color.

src/ShellDocs.Components/Content/PreviewFrame.razor

• Code tab language class swapped from language-markup (Prism's HTML/XML fallback, since Prism ships no razor grammar) to language-razor. Shiki has a real razor grammar — Preview/Code tabs now show proper razor colouring: <Callout> in tag colour, Variant/Title in attribute colour, string values in string colour.

docs/ROADMAP.md

• feat/codeblock-shiki marked ✅ shipped.

Notes

• Package footprint — Shiki loads from esm.sh at runtime (CDN), same delivery model Prism used. When we ship the CLI (shelldocs init), it'll write a <script> tag pointing at the same CDN into the consumer's App.razor. A self-hosted / offline variant can land as a follow-up (feat/shiki-selfhost) — not blocking for Phase 1.
• Deferred: line highlighting via HighlightLines param on a <CodeBlock> component — the roadmap item mentions this but there's no <CodeBlock> API yet; consumers get syntax highlighting from any <pre><code class="language-X"> block in their markdown, which is the same surface Prism exposed. Line-highlight ships in a follow-up branch alongside a proper <CodeBlock Filename="" HighlightLines="" /> API. Not blocking for alpha.
• No test additions — the JS-side highlighter isn't exercised in dotnet test; verification is visual.