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/agent_switch_demo.rs

@@ -1,4 +1,5 @@
 //! agent_switch_demo —— Agent 角色热切换示例。
+//! Required features: cargo run --example agent_switch_demo --features "engine"
 //!
 //! 演示：
 //! 1. 创建 session（绑定 Analyst agent）
@@ -110,6 +111,9 @@ async fn main() {
     };
     println!("\n[verify] turn_index = {turn_index}, agent = {agent_name_owned}");
     assert_eq!(turn_index, 2, "turn_index should be 2 after 2 turns");
-    assert_eq!(agent_name_owned, "reporter", "current agent should be reporter");
+    assert_eq!(
+        agent_name_owned, "reporter",
+        "current agent should be reporter"
+    );
     println!("✓ context preserved across agent switch");
 }