feat(FN-4083): add WAL enforcement, immediate write transactions, and concu

Hardened SQLite concurrent-write safety in `@fusion/core` by auditing WAL enforcement and adding immediate write transactions to prevent stale reads under load, with executor semaphore limit tests in `@fusion/engine` validating the concurrency bounds; added a new `store-concurrent-writes.test.ts` in

Fusion-Task-Id: FN-4083

Fusion-Task-Lineage: adabe51a-a9c4-481a-850d-985c373df51a

packages/engine/src/__tests__/concurrency.test.ts

@@ -199,6 +199,32 @@ describe("AgentSemaphore", () => {
     expect(sem.activeCount).toBe(0);
   });
 
+  it("keeps executor-style write sections within the configured execute concurrency", async () => {
+    const sem = new AgentSemaphore(2);
+    let concurrentWrites = 0;
+    let maxConcurrentWrites = 0;
+
+    const performWrite = (taskId: string) =>
+      sem.run(async () => {
+        concurrentWrites += 1;
+        maxConcurrentWrites = Math.max(maxConcurrentWrites, concurrentWrites);
+        await new Promise((resolve) => setTimeout(resolve, 10));
+        concurrentWrites -= 1;
+        return taskId;
+      }, PRIORITY_EXECUTE);
+
+    const completed = await Promise.all([
+      performWrite("FN-1"),
+      performWrite("FN-2"),
+      performWrite("FN-3"),
+      performWrite("FN-4"),
+    ]);
+
+    expect(completed).toEqual(["FN-1", "FN-2", "FN-3", "FN-4"]);
+    expect(maxConcurrentWrites).toBe(2);
+    expect(sem.activeCount).toBe(0);
+  });
+
   it("integration: shared semaphore limits triage + execution + merge together", async () => {
     const sem = new AgentSemaphore(2);
     let concurrent = 0;

packages/engine/src/__tests__/worktree-db-hydrate.test.ts

@@ -65,9 +65,11 @@ describe("hydrateWorktreeDb", () => {
     const db = new DatabaseSync(join(worktree, ".fusion", "fusion.db"));
     const tasks = (db.prepare("SELECT COUNT(*) as c FROM tasks WHERE id IN ('FN-A','FN-B','FN-C')").get() as any).c;
     const docs = (db.prepare("SELECT COUNT(*) as c FROM task_documents WHERE taskId='FN-B'").get() as any).c;
+    const journalMode = db.prepare("PRAGMA journal_mode").get() as { journal_mode: string };
     db.close();
     expect(tasks).toBe(3);
     expect(docs).toBe(1);
+    expect(journalMode.journal_mode).toBe("wal");
   });
 
   it("no-op when rootDir === worktreePath", async () => {

packages/engine/src/worktree-db-hydrate.ts

@@ -113,8 +113,10 @@ export async function hydrateWorktreeDb({
     }
 
     srcDb = new DatabaseSync(srcDbPath);
+    srcDb.exec("PRAGMA busy_timeout = 5000");
     dstDb = openWorktreeDbWithRecovery(dstDbPath, worktreePath);
 
+    dstDb.exec("PRAGMA busy_timeout = 5000");
     dstDb.exec("PRAGMA journal_mode = WAL");
 
     const srcTaskCols = getColumns(srcDb, "tasks");