fix(SSE): capture Writer before goroutine, use select for channel send

Address PR review feedback:
- Capture w := c.Writer before goroutine start, use w (not c.Writer)
  inside the goroutine to avoid holding *Context reference
- Move channel send into select alongside context cancellation in all
  examples and tests, preventing goroutine leak when client disconnects
  while blocked on unbuffered send

sse_test.go

@@ -68,8 +68,7 @@ func TestEventStreamChanUnblocksOnClientDisconnect(t *testing.T) {
 			select {
 			case <-ctx.Done():
 				return
-			default:
-				eventChan <- Event{Data: "tick"}
+			case eventChan <- Event{Data: "tick"}:
 				time.Sleep(10 * time.Millisecond)
 			}
 		}