test(FN-4754): add engine dashboard diff import boundary guard

Fusion-Task-Id: FN-4754
Fusion-Task-Lineage: 69980e60-c1ad-4e19-bee0-970312669d70

packages/engine/src/__tests__/reliability-interactions/dashboard-diff-boundary.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, it } from "vitest";
+import { readdirSync, readFileSync, statSync } from "node:fs";
+import { dirname, join, relative } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const FORBIDDEN_PATTERNS = [
+  /from\s+["']@fusion\/dashboard["']/g,
+  /import\s*\(\s*["']@fusion\/dashboard["']\s*\)/g,
+  /from\s+["'][^"']*packages\/dashboard\//g,
+  /import\s*\(\s*["'][^"']*packages\/dashboard\//g,
+  /from\s+["'][^"']*(register-session-diff-routes|resolve-diff-base|diff-counts)(\.js)?["']/g,
+  /import\s*\(\s*["'][^"']*(register-session-diff-routes|resolve-diff-base|diff-counts)(\.js)?["']\s*\)/g,
+];
+
+function collectTsFiles(dir: string): string[] {
+  const entries = readdirSync(dir);
+  const files: string[] = [];
+
+  for (const entry of entries) {
+    const fullPath = join(dir, entry);
+    const stat = statSync(fullPath);
+
+    if (stat.isDirectory()) {
+      if (entry === "__tests__") continue;
+      files.push(...collectTsFiles(fullPath));
+      continue;
+    }
+
+    if (!fullPath.endsWith(".ts") && !fullPath.endsWith(".tsx")) continue;
+    if (/\.test\.tsx?$/.test(fullPath)) continue;
+    files.push(fullPath);
+  }
+
+  return files;
+}
+
+function findForbiddenImports(content: string): string[] {
+  const hits: string[] = [];
+  for (const pattern of FORBIDDEN_PATTERNS) {
+    if (pattern.test(content)) {
+      hits.push(pattern.source);
+    }
+    pattern.lastIndex = 0;
+  }
+  return hits;
+}
+
+describe("FN-4754 reliability boundary: engine source must not import dashboard diff display modules", () => {
+  it("keeps engine runtime sources free of dashboard diff/display imports", () => {
+    const testDir = dirname(fileURLToPath(import.meta.url));
+    const engineSrcDir = join(testDir, "..", "..");
+    const files = collectTsFiles(engineSrcDir);
+
+    const offenders: string[] = [];
+    for (const filePath of files) {
+      const content = readFileSync(filePath, "utf8");
+      const matches = findForbiddenImports(content);
+      if (matches.length > 0) {
+        offenders.push(`${relative(engineSrcDir, filePath)} => ${matches.join(", ")}`);
+      }
+    }
+
+    expect(offenders, `Forbidden engine→dashboard imports found:\n${offenders.join("\n")}`).toEqual([]);
+  });
+
+  it("positive control: matcher detects forbidden imports", () => {
+    const sample = `import x from "@fusion/dashboard";\nconst y = await import("../../packages/dashboard/src/routes/diff-counts.js");`;
+    const matches = findForbiddenImports(sample);
+    expect(matches.length).toBeGreaterThan(0);
+  });
+});