Highlight returns SX tree, rendered to HTML/DOM by pipeline

highlight.sx now returns a list of (span :class "..." "text") elements
instead of a string. The rendering pipeline handles the rest:
- Server render-to-html: produces <span class="...">text</span>
- Client render-to-dom: produces DOM span elements
- Aser: serializes spans as SX for client rendering

Key fixes:
- hl-span uses (make-keyword "class") not :class (keywords evaluate
  to strings in list context)
- render-sx-tokens returns flat list of spans (no wrapper)
- hl-escape is identity (no escaping needed for tree values)
- highlight.sx added to browser bundle + platform loader
- ~docs/code renders src directly as child of pre

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/browser/bundle.sh

@@ -34,8 +34,9 @@ cp "$ROOT/web/deps.sx"           "$DIST/sx/"
 cp "$ROOT/web/router.sx"         "$DIST/sx/"
 cp "$ROOT/web/page-helpers.sx"   "$DIST/sx/"
 
-# 3b. Freeze scope (signal persistence)
+# 3b. Freeze scope (signal persistence) + highlight (syntax coloring)
 cp "$ROOT/lib/freeze.sx"         "$DIST/sx/"
+cp "$ROOT/lib/highlight.sx"      "$DIST/sx/"
 
 # 4. Bytecode compiler + VM
 cp "$ROOT/lib/bytecode.sx"       "$DIST/sx/"