Composed-tree shadow DOM capture, sized invisible spacers, track re-anchoring, block fill-width slides, javascript: href sanitization

- Walker serializes the composed (flattened) tree: open shadow roots walk
  as host children with <slot> replaced by its assigned light nodes (or
  fallback), so custom-element content (product cards, web components) is
  captured instead of arriving childless
- In-flow visibility:hidden nodes with a nonzero border box survive the
  prune as sized invisible placeholders - ghost spacers carry load-bearing
  geometry
- When pruning drops leading in-flow children of a horizontally-translated
  track, the baked translateX is re-anchored by their aggregate margin-box
  width (settled carousel tracks return to origin instead of pushing real
  content off-screen); animation-owned transforms unaffected
- Circular-shrink slide guard counts block-level fill children as
  width-deriving regardless of wAuto (block auto-width IS fill), pinning
  library-sized slide widths uniformly
- javascript: hrefs emit as "#" and the link gate normalizes both sides

382 tests pass (18 new), typecheck clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

compiler/src/generate/app.ts

@@ -369,11 +369,20 @@ export function propsList(node: IRNode, assetMap: Map<string, string>, sourceUrl
       if (kept.length === 0) continue;
       value = kept.join(", ");
     } else if (key === "href") {
-      // Preserve in-page anchors. For multi-route sites a linkRewrite maps internal
-      // links to the generated clone routes (and collapsed-collection links to their
-      // representative); otherwise absolutize so it never 404s inside the clone
-      // (external navigation is allowed by the rubric).
-      if (!value.startsWith("#")) value = ctx?.linkRewrite ? ctx.linkRewrite(value) : resolveUrl(value, sourceUrl);
+      // A `javascript:*` href (announcement bars, JS-driven buttons authored as links) is a
+      // script trigger, not a navigable URL. React refuses to render one: it rewrites the
+      // attribute to a long `javascript:throw new Error('React has blocked a javascript: URL…')`
+      // string, which no longer matches the source href in the link gate. The behaviour is
+      // script we don't reproduce anyway, so emit an inert `#` and keep the anchor navigable-inert.
+      if (/^\s*javascript:/i.test(value)) {
+        value = "#";
+      } else if (!value.startsWith("#")) {
+        // Preserve in-page anchors. For multi-route sites a linkRewrite maps internal
+        // links to the generated clone routes (and collapsed-collection links to their
+        // representative); otherwise absolutize so it never 404s inside the clone
+        // (external navigation is allowed by the rubric).
+        value = ctx?.linkRewrite ? ctx.linkRewrite(value) : resolveUrl(value, sourceUrl);
+      }
     }
 
     let reactName = isCustom ? key : (ATTR_RENAME[key] ?? key);