feat: ASR 从双向流式切换为流式输入模式（bigmodel_nostream）

- endpoint 从 bigmodel_async 改为 bigmodel_nostream
- 二进制协议去掉 sequence 字段，初始请求和音频帧均不带序号
- 最后一帧使用 FlagLastNoSeq 标志
- RequestMeta 新增 result_type=single、end_window_size=400
- ShowUtterances 关闭（nostream 模式不需要）
- readLoop 简化：nostream 模式下直接返回 final 结果

internal/asr/client.go

@@ -5,7 +5,6 @@ import (
 	"log/slog"
 	"net/http"
 	"sync"
-	"sync/atomic"
 	"time"
 
 	"github.com/fasthttp/websocket"
@@ -14,7 +13,7 @@ import (
 )
 
 const (
-	doubaoEndpoint = "wss://openspeech.bytedance.com/api/v3/sauc/bigmodel_async"
+	doubaoEndpoint = "wss://openspeech.bytedance.com/api/v3/sauc/bigmodel_nostream"
 	writeTimeout   = 10 * time.Second
 	readTimeout    = 30 * time.Second
 )
@@ -30,7 +29,6 @@ type Config struct {
 type Client struct {
 	cfg     Config
 	conn    *websocket.Conn
-	seq     atomic.Int32
 	mu      sync.Mutex
 	closed  bool
 	closeCh chan struct{}
@@ -74,11 +72,12 @@ func Dial(cfg Config, resultCh chan<- wsMsg.ServerMsg) (*Client, error) {
 			EnableITN:      true,
 			EnablePUNC:     true,
 			EnableDDC:      true,
-			ShowUtterances: true,
+			ShowUtterances: false,
+			ResultType:     "single",
+			EndWindowSize:  400,
 		},
 	}
-	c.seq.Store(1)
-	data, err := EncodeFullClientRequest(req, c.seq.Load())
+	data, err := EncodeFullClientRequest(req)
 	if err != nil {
 		conn.Close()
 		return nil, fmt.Errorf("encode full request: %w", err)
@@ -100,8 +99,7 @@ func (c *Client) SendAudio(pcm []byte, last bool) error {
 	if c.closed {
 		return fmt.Errorf("client closed")
 	}
-	seq := c.seq.Add(1)
-	data, err := EncodeAudioFrame(seq, pcm, last)
+	data, err := EncodeAudioFrame(pcm, last)
 	if err != nil {
 		return fmt.Errorf("encode audio: %w", err)
 	}
@@ -134,20 +132,10 @@ func (c *Client) readLoop(resultCh chan<- wsMsg.ServerMsg) {
 			resultCh <- wsMsg.ServerMsg{Type: wsMsg.MsgError, Message: resp.ErrMsg}
 			return
 		}
-		// Determine if this is a final result by checking utterances
-		isFinal := false
+		// nostream mode: result comes after last audio packet or >15s
 		text := resp.Text
-		for _, u := range resp.Utterances {
-			if u.Definite {
-				isFinal = true
-				text = u.Text
-				break
-			}
-		}
-		if isFinal {
+		if text != "" {
 			resultCh <- wsMsg.ServerMsg{Type: wsMsg.MsgFinal, Text: text}
-		} else if text != "" {
-			resultCh <- wsMsg.ServerMsg{Type: wsMsg.MsgPartial, Text: text}
 		}
 		if resp.IsLast {
 			return