docs: Add Mermaid diagrams for architecture visualization

- Added flowchart showing thread pool architecture
- Added sequence diagram for work-stealing flow
- Added state diagram for task lifecycle

Author: Rishabh Rohil

README.md

@@ -110,23 +110,79 @@ target_link_libraries(your_target PRIVATE threadpool)
 
 ## Architecture
 
+```mermaid
+flowchart TB
+    subgraph Client[Client Application]
+        Submit[submit task]
+        Future[get future]
+    end
+
+    subgraph Pool[ThreadPool]
+        subgraph Workers[Worker Threads]
+            W1[Worker 1]
+            W2[Worker 2]
+            WN[Worker N]
+        end
+        
+        subgraph Queues[Task Queues]
+            GQ[Global Priority Queue]
+            LQ1[Local Queue 1]
+            LQ2[Local Queue 2]
+            LQN[Local Queue N]
+        end
+    end
+
+    Submit --> GQ
+    GQ --> W1 & W2 & WN
+    W1 --> LQ1
+    W2 --> LQ2
+    WN --> LQN
+    
+    LQ1 -.->|steal| W2
+    LQ2 -.->|steal| WN
+    LQN -.->|steal| W1
+    
+    W1 & W2 & WN --> Future
+```
+
+### Work-Stealing Flow
+
+```mermaid
+sequenceDiagram
+    participant C as Client
+    participant GQ as Global Queue
+    participant W1 as Worker 1
+    participant W2 as Worker 2
+    participant LQ1 as Local Queue 1
+
+    C->>GQ: submit(task)
+    GQ->>W1: pop task
+    W1->>W1: execute task
+    
+    Note over W2: Queue empty
+    W2->>LQ1: steal task
+    LQ1->>W2: return task
+    W2->>W2: execute stolen task
+    
+    W1->>C: future.get() returns
 ```
-┌─────────────────────────────────────────────────────────────┐
-│                        ThreadPool                            │
-├─────────────────────────────────────────────────────────────┤
-│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐         │
-│  │  Worker 1   │  │  Worker 2   │  │  Worker N   │         │
-│  │ ┌─────────┐ │  │ ┌─────────┐ │  │ ┌─────────┐ │         │
-│  │ │Local Q  │◄┼──┼─┤ STEAL   │◄┼──┼─┤Local Q  │ │         │
-│  │ └─────────┘ │  │ └─────────┘ │  │ └─────────┘ │         │
-│  └──────┬──────┘  └──────┬──────┘  └──────┬──────┘         │
-│         └────────────────┼────────────────┘                 │
-│                          ▼                                  │
-│  ┌─────────────────────────────────────────────────────┐   │
-│  │             Global Priority Queue                    │   │
-│  │  [P:0] [P:1] [P:2] [P:5] [P:10] ← Higher priority   │   │
-│  └─────────────────────────────────────────────────────┘   │
-└─────────────────────────────────────────────────────────────┘
+
+---
+
+### Task Lifecycle
+
+```mermaid
+stateDiagram-v2
+    [*] --> Submitted: submit()
+    Submitted --> Queued: Added to queue
+    Queued --> Running: Worker picks up
+    Running --> Completed: Success
+    Running --> Failed: Exception
+    Completed --> [*]: future.get()
+    Failed --> [*]: future.get() throws
+    
+    Queued --> Cancelled: shutdown_now()
+    Cancelled --> [*]
 ```
 
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