feat(FN-4180): compact room transcript context to reduce token usage

Adds room transcript context compaction (FN-4180) to the dashboard chat layer — widens the fetch window then compacts older messages into a summary window to reduce token overhead while keeping full context recent. Includes a new 127-line test suite covering the compaction logic, a small fix to reus

Fusion-Task-Id: FN-4180

Fusion-Task-Lineage: 9e7e3a32-f2da-4c86-871b-37891f747561

packages/engine/src/__tests__/merger-overlap-guard.test.ts

@@ -173,6 +173,10 @@ function cleanupTempDir(dir?: string): void {
 
     sleepSync(attempt * 25);
   }
+
+  if (existsSync(dir)) {
+    throw new Error(`failed to clean temp dir: ${dir}`);
+  }
 }
 
 afterAll(() => {

packages/engine/src/__tests__/merger-staging-allowlist.test.ts

@@ -87,19 +87,29 @@ const STUB_SETTINGS = {
 
 const createdDirs = new Set<string>();
 
+function sleepSync(ms: number): void {
+  Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, ms);
+}
+
 function cleanupTempDir(dir?: string): void {
   if (!dir) return;
   createdDirs.delete(dir);
-  for (let attempt = 1; attempt <= 3; attempt += 1) {
+  for (let attempt = 1; attempt <= 5; attempt += 1) {
     try {
       if (!existsSync(dir)) return;
       rmSync(dir, { recursive: true, force: true });
-      return;
+      if (!existsSync(dir)) return;
     } catch (error) {
-      if (attempt === 3) {
+      if (attempt === 5) {
         throw error;
       }
     }
+
+    sleepSync(attempt * 25);
+  }
+
+  if (existsSync(dir)) {
+    throw new Error(`failed to clean temp dir: ${dir}`);
   }
 }