feat: free mode random walk + pause wake lock release

State machine (张明远):
- Add select_random_free_state() for FreeMode states without
  next_state/next_states config
- Weighted random selection across all FreeMode states
- Matches old hologram_player_rust behavior

Video plugin (赵雨薇):
- Send ScreenLockRequest(false) to screen plugin on Pause
- Send ScreenLockRequest(true) to screen plugin on Play/Resume
- Closes the pause-wake-lock gap vs old version

cargo check: 0 warnings, cargo test: 22/22 passed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plugins/video/mod.rs

@@ -147,9 +147,25 @@ impl Plugin for VideoPlugin {
                 match command {
                     PlayerCommand::Play => {
                         processor.play()?;
+                        // 恢复播放时重新获取防息屏锁
+                        if let Some(ctx) = &self.ctx {
+                            let _ = ctx.tx.send(Envelope {
+                                from: self.id(),
+                                to: Destination::Plugin("screen"),
+                                message: Message::ScreenLockRequest(true),
+                            });
+                        }
                     }
                     PlayerCommand::Pause => {
                         processor.pause();
+                        // 暂停时释放防息屏锁
+                        if let Some(ctx) = &self.ctx {
+                            let _ = ctx.tx.send(Envelope {
+                                from: self.id(),
+                                to: Destination::Plugin("screen"),
+                                message: Message::ScreenLockRequest(false),
+                            });
+                        }
                     }
                     PlayerCommand::Next => {
                         processor.next_video()?;