feat(FN-3710): retry overlap-class cluster create failures in distributed t

Adds retry logic for distributed task ID overlap conflicts across the cluster, hardening the ID allocator with exponential-backoff recovery, wiring the retry into task workflow registration, and documenting the behavior in architecture docs. Minor CSS tokenization on the Mailbox and Plugin Manager m

Fusion-Task-Id: FN-3710

packages/core/src/__tests__/distributed-task-id.test.ts

@@ -84,6 +84,28 @@ describe("distributed-task-id allocator", () => {
     expect(state.nextSequence).toBe(3702);
   });
 
+  it("skips stale overlapping nextSequence values and reserves the next free id", async () => {
+    const db = new Database("/tmp/fusion-test", { inMemory: true });
+    db.init();
+    const now = new Date().toISOString();
+    db.prepare(
+      "INSERT INTO tasks (id, description, \"column\", createdAt, updatedAt) VALUES (?, '', 'todo', ?, ?)",
+    ).run("FN-002", now, now);
+    db.prepare(
+      "INSERT INTO distributed_task_id_state (prefix, nextSequence, committedClusterTaskCount, lastCommittedTaskId, updatedAt) VALUES (?, ?, ?, ?, ?)",
+    ).run("FN", 2, 1, "FN-001", now);
+
+    const allocator = createDistributedTaskIdAllocator(db);
+    const reservation = await allocator.reserveDistributedTaskId({ prefix: "FN", nodeId: "node-a" });
+
+    expect(reservation.taskId).toBe("FN-003");
+    expect(reservation.sequence).toBe(3);
+
+    const state = await allocator.getDistributedTaskIdState({ prefix: "FN" });
+    expect(state.nextSequence).toBe(4);
+    expect(state.committedClusterTaskCount).toBe(1);
+  });
+
   it("state reports committed count independently from nextSequence", async () => {
     const { allocator } = createAllocator();
     const first = await allocator.reserveDistributedTaskId({ prefix: "FN", nodeId: "node-a" });

packages/core/src/distributed-task-id.ts

@@ -81,6 +81,22 @@ export function createDistributedTaskIdAllocator(db: Database): DistributedTaskI
     return result.changes ?? 0;
   };
 
+  const taskIdExists = (prefix: string, sequence: number): boolean => {
+    const taskId = formatDistributedTaskId(prefix, sequence);
+    const existsInTable = (table: string): boolean => {
+      try {
+        const row = db
+          .prepare(`SELECT 1 as found FROM ${table} WHERE id = ? LIMIT 1`)
+          .get(taskId) as { found?: number } | undefined;
+        return row?.found === 1;
+      } catch {
+        return false;
+      }
+    };
+
+    return existsInTable("tasks") || existsInTable("archivedTasks");
+  };
+
   const ensureStateRow = (prefix: string): void => {
     // Seed nextSequence past any pre-existing task ID for this prefix. Without
     // this, projects whose tasks were originally allocated through
@@ -161,7 +177,11 @@ export function createDistributedTaskIdAllocator(db: Database): DistributedTaskI
             )
             .get(prefix) as { nextSequence: number; committedClusterTaskCount: number };
 
-          const sequence = state.nextSequence;
+          let sequence = state.nextSequence;
+          while (taskIdExists(prefix, sequence)) {
+            sequence += 1;
+          }
+
           const taskId = formatDistributedTaskId(prefix, sequence);
           const reservationId = randomUUID();