fix(state_portal): make shutdown() idempotent to prevent duplicate log lines (#22)

shutdown() fired twice on exit (explicit call in main.rs + Drop impl), causing "Total: N frames" and "StatePortal shutdown complete" to log twice 23us apart. Add `shutdown_started: bool` guard at function entry.

Plain bool (not AtomicBool) because &mut self already grants exclusive access. Guard is set BEFORE cleanup so Drop re-entry during panic unwinding is suppressed.

Includes scripts/test_shutdown_idempotency.sh as a live regression test (requires Wayland session). Verified PASS on KWin: both lines print exactly once.

src/state_portal.rs

@@ -66,6 +66,7 @@ pub struct StatePortal {
     last_fillable_frame: Option<CpuNv12Frame>, // cached last frame for filler duplication
     next_filler_at: Option<Instant>,       // when to send next filler frame
     filler_frames_sent: u64,
+    shutdown_started: bool, // idempotency guard; plain bool because &mut self is exclusive (not AtomicBool)
 }
 
 impl StatePortal {
@@ -112,6 +113,7 @@ impl StatePortal {
             last_fillable_frame: None,
             next_filler_at: None,
             filler_frames_sent: 0,
+            shutdown_started: false,
         })
     }
 
@@ -586,10 +588,15 @@ impl StatePortal {
         Ok(())
     }
 
-    /// 关闭状态：刷新编码器并清理资源
+    /// 关闭状态：刷新编码器并清理资源（幂等）。
     ///
-    /// 使用 `enc.take()` 确保编码器只被 flush 一次，即使多次调用也安全（幂等）。
+    /// `shutdown_started` 守卫在清理之前置位——防止 panic 时 `Drop` 重入 unwinding。
     pub fn shutdown(&mut self) {
+        if self.shutdown_started {
+            return;
+        }
+        self.shutdown_started = true;
+
         self.last_fillable_frame = None;
         // 1. Stop encode thread (drops webrtc_tx → signals WebRTC thread to exit)
         if let Some(mut enc_thread) = self.enc_thread.take() {