feat(ui): highlight source server-side with a near-monochrome Shiki theme (CG-43)

The viewer's code block stops lexing with a hand-rolled dialect table and
reads real TextMate grammars instead, run once in `/api/source`.

Three things make that safe to depend on:

* Highlighting never fails a request. A missing grammar, an oversized
  slice, an ESM import that did not resolve — every one of them answers
  `engine: 'plain'` with a reason and the source still goes out.
* Identifiers survive whatever token boundaries a grammar chose. Every
  code token is split into identifier runs before it goes on the wire, so
  the graph's call-site overlay claims a token the highlighter produced
  rather than re-cutting the line. `assignRefs` now matches on a token's
  text rather than on the class a grammar gave it, so a language that
  scopes type names as `storage.type` still links.
* The theme classifies rather than colours: its foregrounds are sentinels
  the server maps back to class names, and the viewer paints them from
  CSS custom properties — one token stream serves light and dark with no
  refetch, and the ramp lives only in app.css.

Comments move from --ink-3 to a new --code-comment. --ink-3 measures
3.46:1 on paper and 3.00:1 on the hot-line tint, both under AA for 12.5px
text; --code-comment is the smallest step along the same ramp that clears
4.5:1 on every background a code line can have, and stays quieter than
the strings and numbers above it.

Shipping: @shikijs/core and @shikijs/engine-javascript are runtime
dependencies (no wasm, no native module); @shikijs/langs stays a
devDependency and `npm run build:textmate` writes only the closure the
engine's 40-odd languages reach — 56 grammars, 2.6 MB, against 11 MB for
all 722. check-ui-build.mjs asserts the tree after every build and inside
every release archive.

ui/src/lib/api.ts

@@ -11,6 +11,8 @@
  * carries the server's own sentence instead of "Failed to fetch".
  */
 
+import type { WireHighlight } from './highlight';
+
 /* ---------------------------------------------------------------- shapes -- */
 
 export type NodeKind = string;
@@ -153,6 +155,13 @@ export interface WireSource {
   lines?: string[];
   truncated?: boolean;
   reason?: string;
+  /**
+   * The same lines, classified by the server's TextMate grammars — one entry
+   * per line, each a list of `[classId, text]` pairs indexed into `classes`.
+   * Absent whenever `lines` is, and `engine: 'plain'` whenever no grammar
+   * covers the file. See `lib/highlight.ts`.
+   */
+  highlight?: WireHighlight;
 }
 
 export interface WireBlastScale {