docs: 更新 README feature 表 + 升级指南 + 示例注释 + roadmap 同步

- README 添加 feature 组合表 + 模块级 features 清单 + 升级指南
- 18 个 example 顶部添加 Required features 注释
- roadmap.md 和 roadmap-v0.3.2.md 同步 Phase 26-27 完成状态
- cargo fmt 全量格式化（修复预存格式问题，CI format job 可通过）

examples/dispatch_stream_demo.rs

@@ -1,4 +1,5 @@
 //! dispatch_stream_demo —— 流式子代理调度示例。
+//! Required features: cargo run --example dispatch_stream_demo --features "engine"
 //!
 //! 演示：
 //! 1. 创建父 session
@@ -92,7 +93,11 @@ async fn main() {
                 saw_stream_count += 1;
             }
             SubTaskStreamEvent::Completed(r) => {
-                println!("  → Completed(child_id={}, {} tokens)", &r.child_id[..20], r.usage.total().total_tokens);
+                println!(
+                    "  → Completed(child_id={}, {} tokens)",
+                    &r.child_id[..20],
+                    r.usage.total().total_tokens
+                );
                 completed = Some(r);
                 break;
             }
@@ -114,7 +119,10 @@ async fn main() {
     let child_guard = child_session.lock().await;
     let child_turn_index = child_guard.turn_index();
     drop(child_guard);
-    assert_eq!(child_turn_index, 1, "turn_index should increment after finalize");
+    assert_eq!(
+        child_turn_index, 1,
+        "turn_index should increment after finalize"
+    );
     println!("[4] child session turn_index = {child_turn_index} (finalize works)");
 
     println!("\n✓ dispatch_stream completed successfully");