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/package.json

@@ -20,11 +20,13 @@
   },
   "license": "BSD-3-Clause",
   "dependencies": {
+    "@shikijs/langs": "^4.3.1",
     "clsx": "^2.0.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-markdown": "^10.1.0",
-    "remark-gfm": "^4.0.1"
+    "remark-gfm": "^4.0.1",
+    "shiki": "^4.3.1"
   },
   "devDependencies": {
     "@deepseek-ai/dsh-invariants": "workspace:^",