feat(web): shiki syntax highlighting for code surfaces

One highlighter for the client: a synchronous fine-grained shiki core
(JS regex engine, no WASM) in ui-primitives with an explicit grammar
allowlist (typescript, shellscript, json — aliases resolve, unknown
languages take a geometry-identical plain arm). The shared CodeBlock
component owns both arms; markdown fences, the run_code expanded
program body (typescript), and the details panel Input (json) all
route through it. Token colors live in a new ui-theme shiki.css sheet
as --shiki-* custom properties (light/dark blocks), wired through the
shell's base.css chain — tokens-only styling holds; shiki's generated
span tree is the sanctioned innerHTML path (static output, no user
HTML). jsdom specs pin token spans, aliases, both fallbacks, and the
fence route; the built-bundle snapshot asserts the highlighted program
under the code row.

packages/client/ui-primitives/src/markdown/highlight.ts

@@ -0,0 +1,68 @@
+/**
+ * The client's ONE syntax highlighter: a synchronous fine-grained shiki core
+ * (JavaScript regex engine — no oniguruma WASM, bundle-friendly) with an
+ * explicit grammar allowlist and a CSS-variables theme. Colors live in the
+ * theme package's token sheets as `--shiki-*` custom properties (light and
+ * dark blocks), never here — the repo's tokens-only styling rule.
+ *
+ * Grammars are the set the harness actually renders: TypeScript programs
+ * (`run_code` bodies; TS pulls in JS via grammar embedding), shell commands,
+ * and JSON payloads. An unknown or absent language falls back to plain text
+ * (no highlighting, still monospace) — never an error.
+ */
+
+import { createHighlighterCoreSync, createCssVariablesTheme } from 'shiki/core'
+import { createJavaScriptRegexEngine } from 'shiki/engine/javascript'
+import langTs from '@shikijs/langs/typescript'
+import langBash from '@shikijs/langs/shellscript'
+import langJson from '@shikijs/langs/json'
+import type { HighlighterCore } from 'shiki/core'
+
+/** Language ids (and aliases) the singleton registers; everything else renders plain. */
+const LANG_ALIASES: Record<string, string> = {
+  typescript: 'typescript',
+  ts: 'typescript',
+  tsx: 'typescript',
+  javascript: 'typescript',
+  js: 'typescript',
+  shellscript: 'shellscript',
+  bash: 'shellscript',
+  sh: 'shellscript',
+  shell: 'shellscript',
+  zsh: 'shellscript',
+  json: 'json',
+  jsonc: 'json',
+}
+
+/** All token colors resolve through `--shiki-*` custom properties (theme package sheets). */
+const cssVariablesTheme = createCssVariablesTheme({
+  name: 'css-variables',
+  variablePrefix: '--shiki-',
+  fontStyle: true,
+})
+
+let singleton: HighlighterCore | undefined
+
+/** The lazily-created synchronous highlighter (one instance per document). */
+function highlighter(): HighlighterCore {
+  singleton ??= createHighlighterCoreSync({
+    themes: [cssVariablesTheme],
+    langs: [langTs, langBash, langJson],
+    engine: createJavaScriptRegexEngine({ forgiving: true }),
+  })
+  return singleton
+}
+
+/**
+ * Highlight `code` into shiki's HTML (a single `<pre class="shiki">` tree)
+ * when `lang` maps to a registered grammar; `undefined` means the caller
+ * renders its plain fallback.
+ * @param code - the source text.
+ * @param lang - the language hint (a markdown fence info string or a fixed caller id).
+ * @returns the highlighted HTML, or `undefined` for unknown languages.
+ */
+export function highlightToHtml(code: string, lang: string | undefined): string | undefined {
+  const resolved = lang === undefined ? undefined : LANG_ALIASES[lang.toLowerCase()]
+  if (resolved === undefined) return undefined
+  return highlighter().codeToHtml(code, { lang: resolved, theme: 'css-variables' })
+}