fix(frontend): cap schema-linking graph on promoted count + signals in table Perché

- Cap counts promoted candidates only (was all candidates); reuse promoted list (DRY)
- New test: 10 promoted + 50 excluded must not cap, graph stays available
- Extract reasonFor() helper; table Perché falls back to signal keys like the flowchart

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/viewers/SchemaLinkingViewer.test.tsx

@@ -55,3 +55,30 @@ test("(c) oversized linking (>45) shows cap notice", () => {
     screen.getByText(/troppi elementi per il grafico/i)
   ).toBeInTheDocument();
 });
+
+test("(d) ~10 promoted + ~50 excluded does NOT cap; graph is available", async () => {
+  const mixedLinking = {
+    question: "caso misto",
+    candidates: [
+      ...Array.from({ length: 10 }, (_, i) => ({
+        kind: "table" as const,
+        name: `kept_${i}`,
+        decision: "promoted" as const,
+      })),
+      ...Array.from({ length: 50 }, (_, i) => ({
+        kind: "table" as const,
+        name: `dropped_${i}`,
+        decision: "excluded" as const,
+      })),
+    ],
+    joins: [],
+    excluded: [],
+    open_questions: [],
+  };
+  render(<SchemaLinkingViewer linking={mixedLinking} />);
+  expect(
+    screen.queryByText(/troppi elementi per il grafico/i)
+  ).not.toBeInTheDocument();
+  // Grafico (mermaid) view is the default and renders the mocked svg.
+  await waitFor(() => expect(screen.getByTestId("mm")).toBeInTheDocument());
+});