fix(stream): OnClose 可在返回前注册，消除 Inject 循环等待死锁

旧实现 OnClose 先等待 ready，但 Inject 只有业务方法返回后才发生：业务同步 OnClose → 等 Inject；框架等业务返回 → 永久死锁，流首帧永不输出。改为仅持锁注册回调，不等待 channel 注入；已关闭场景锁外立即回调，避免重入自锁。新增两条回归测试。

stream_onclose_test.go

@@ -0,0 +1,46 @@
+package fun
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// OnClose 必须允许业务方法在 return *Stream 之前同步注册；旧实现等待 Inject，
+// 而 Inject 只有 invoke 在业务方法返回后才调用，形成精确死锁：
+//
+//	business OnClose -> wait ready -> invoke wait business return -> never Inject.
+func TestStreamOnCloseBeforeInjectDoesNotBlock(t *testing.T) {
+	st := &Stream{}
+	registered := make(chan struct{})
+	var called atomic.Int32
+	go func() {
+		st.OnClose(func() { called.Add(1) })
+		close(registered)
+	}()
+	select {
+	case <-registered:
+		// expected: registration is independent from channel injection
+	case <-time.After(time.Second):
+		t.Fatal("OnClose blocked before Inject (business method would deadlock)")
+	}
+
+	ch := make(chan any)
+	done := make(chan struct{})
+	st.Inject(ch, done)
+	st.Close()
+	if got := called.Load(); got != 1 {
+		t.Fatalf("callback called %d times, want 1", got)
+	}
+}
+
+func TestStreamOnCloseAfterAlreadyClosedRunsImmediately(t *testing.T) {
+	st := &Stream{}
+	st.Inject(make(chan any), make(chan struct{}))
+	st.Close()
+	var called atomic.Int32
+	st.OnClose(func() { called.Add(1) })
+	if got := called.Load(); got != 1 {
+		t.Fatalf("callback called %d times, want 1", got)
+	}
+}