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/core/src/store.ts

@@ -1740,7 +1740,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     task: Task,
     auditInput?: RunAuditEventInput,
   ): Promise<void> {
-    this.db.transaction(() => {
+    this.db.transactionImmediate(() => {
       // Upsert the task
       this.upsertTaskWithFtsRecovery(task);
 
@@ -4137,7 +4137,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       metadata: input.metadata,
     };
 
-    this.db.transaction(() => {
+    this.db.transactionImmediate(() => {
       this.db.prepare(`
         INSERT INTO runAuditEvents (
           id, timestamp, taskId, agentId, runId, domain, mutationType, target, metadata