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/CodeBlock.module.css

@@ -0,0 +1,27 @@
+/* One code-block geometry for highlighted and plain arms: the shiki <pre>
+   and the fallback <pre> draw identically except for token colors. */
+
+.block :where(pre) {
+  margin: 0;
+  padding: 8px 10px;
+  border-radius: 8px;
+  overflow-x: auto;
+  background: var(--dsw-alias-markdown-code-block);
+  font: var(--dsw-font-markdown-code-block);
+}
+
+/* Shiki inlines its theme background var; route it to the repo token. */
+.block :where(pre.shiki) {
+  background: var(--dsw-alias-markdown-code-block) !important;
+}
+
+.block :where(pre) code {
+  font: inherit;
+  background: none;
+  padding: 0;
+}
+
+.plain {
+  color: var(--dsw-alias-label-primary);
+  white-space: pre;
+}