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-theme/src/styles/shiki.css

@@ -0,0 +1,31 @@
+/* Syntax-highlight token palette: the values behind shiki's css-variables
+   theme (--shiki-* custom properties emitted by the ui-primitives CodeBlock).
+   Light values on :root, dark overrides on the body attribute — the same
+   cascade as every other token sheet. Background/foreground deliberately
+   alias the markdown code-block tokens so highlighted and plain blocks agree. */
+
+:root {
+  --shiki-foreground: var(--dsw-alias-label-primary);
+  --shiki-background: var(--dsw-alias-markdown-code-block);
+  --shiki-token-constant: #1c7ed6;
+  --shiki-token-string: #2f9e44;
+  --shiki-token-comment: #868e96;
+  --shiki-token-keyword: #d6336c;
+  --shiki-token-parameter: #e8590c;
+  --shiki-token-function: #6741d9;
+  --shiki-token-string-expression: #2b8a3e;
+  --shiki-token-punctuation: #495057;
+  --shiki-token-link: #1971c2;
+}
+
+body[data-ds-dark-theme] {
+  --shiki-token-constant: #4dabf7;
+  --shiki-token-string: #69db7c;
+  --shiki-token-comment: #adb5bd;
+  --shiki-token-keyword: #faa2c1;
+  --shiki-token-parameter: #ffa94d;
+  --shiki-token-function: #b197fc;
+  --shiki-token-string-expression: #8ce99a;
+  --shiki-token-punctuation: #ced4da;
+  --shiki-token-link: #74c0fc;
+}