feat(video): 实现 FreeMode 状态随机游走

- 修改 select_next_state()：FreeMode 无 next_state/next_states 时按权重随机选择 FreeMode 状态
- 新增 select_random_free_state() 方法实现权重随机算法
- InteractiveMode 保持原行为（停留当前状态）
- 新增单元测试验证随机游走和 InteractiveMode 行为
- 所有测试通过（24/24）

旧版行为回补完成。

src/plugins/video/state_machine.rs

@@ -538,7 +538,7 @@ mod tests {
 
     #[test]
     fn interactive_mode_stays_in_same_state_when_no_next_state_configured() {
-        let mut machine = StateMachine::new(config_with_states([StateConfig {
+        let states = vec![StateConfig {
             name: "interactive".to_string(),
             mode: StateMode::InteractiveMode,
             sequence: vec![step("interactive")],
@@ -548,7 +548,20 @@ mod tests {
             weight: 1.0,
             defer_triggers: false,
             ignore_triggers: false,
-        }]));
+        }];
+
+        let states_map = states
+            .into_iter()
+            .map(|state| {
+                let name = state.name.clone();
+                (name, state)
+            })
+            .collect::<HashMap<_, _>>();
+
+        let mut machine = StateMachine::new(StateMachineConfig {
+            initial_state: "interactive".to_string(),
+            states: states_map,
+        });
         machine.start().expect("state machine should start");
 
         assert!(!machine