docs: clarify reverse proxy compatibility behavior

Document BufferPool usage and explain why trailer fallback and disconnect compatibility logic intentionally mirror the standard library reverse proxy. Add a regression test covering unannounced trailer forwarding so that proxy trailer behavior stays aligned with Go's semantics.

docs/reverse-proxy.md

@@ -110,6 +110,40 @@ r.ANY("/api/*path", touka.ReverseProxy(touka.ReverseProxyConfig{
 
 对于 SSE 和无 `Content-Length` 的流式响应，Touka 会自动立即刷新，不依赖该配置。
 
+### `BufferPool`
+
+可选。用于为响应体复制过程提供可复用的字节缓冲区，以减少大响应或高并发代理场景下的临时内存分配。
+
+如果留空，Touka 会在复制响应体时按需分配默认缓冲区。
+
+```go
+type bytePool struct {
+    pool sync.Pool
+}
+
+func (p *bytePool) Get() []byte {
+    if buf, ok := p.pool.Get().([]byte); ok {
+        return buf
+    }
+    return make([]byte, 32*1024)
+}
+
+func (p *bytePool) Put(buf []byte) {
+    if cap(buf) >= 32*1024 {
+        p.pool.Put(buf[:32*1024])
+    }
+}
+
+proxyPool := &bytePool{}
+
+r.ANY("/api/*path", touka.ReverseProxy(touka.ReverseProxyConfig{
+    Target:     target,
+    BufferPool: proxyPool,
+}))
+```
+
+通常只有在您已经观察到明显的分配压力，或代理的响应体较大、吞吐较高时，才需要专门配置它。
+
 ### `ModifyRequest`
 
 在请求真正发往后端前，对出站请求做最后修改。

reverseproxy.go

@@ -359,6 +359,10 @@ func (p *reverseProxyHandler) ServeHTTP(c *Context) {
 		c.Writer.Flush()
 	}
 
+	// Keep the stdlib-compatible fallback here.
+	// If the backend only exposes additional trailer keys after the body has been
+	// fully read, the trailer map can grow and those values must be written using
+	// the TrailerPrefix form instead of the pre-announced bare header keys.
 	if len(res.Trailer) == announcedTrailers {
 		reverseProxyCopyHeader(c.Writer.Header(), res.Trailer)
 		return
@@ -378,6 +382,11 @@ func (p *reverseProxyHandler) requestContext(c *Context) (context.Context, conte
 		return ctx, func() {}
 	}
 
+	// Follow the same compatibility path as net/http/httputil.ReverseProxy:
+	// request contexts are normally cancelable, but middleware can still replace
+	// c.Request with one backed by context.Background/TODO or another context with
+	// a nil Done channel. In that case CloseNotifier still provides disconnect
+	// propagation for the upstream round trip.
 	rawWriter := reverseProxyBaseResponseWriter(c.Writer)
 	cn, ok := rawWriter.(http.CloseNotifier)
 	if !ok {

reverseproxy_test.go

@@ -187,6 +187,43 @@ func TestReverseProxyCustomErrorHandler(t *testing.T) {
 	}
 }
 
+func TestReverseProxyUnannouncedTrailerForwarding(t *testing.T) {
+	t.Helper()
+
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set(http.TrailerPrefix+"X-Unannounced-Trailer", "later")
+		w.WriteHeader(http.StatusOK)
+		_, _ = io.WriteString(w, "streamed")
+	}))
+	defer backend.Close()
+
+	target, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatalf("parse target: %v", err)
+	}
+
+	engine := New()
+	engine.GET("/trailers", ReverseProxy(ReverseProxyConfig{Target: target}))
+
+	rr := PerformRequest(engine, http.MethodGet, "/trailers", nil, nil)
+	resp := rr.Result()
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+	_ = resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unexpected status: %d", resp.StatusCode)
+	}
+	if string(body) != "streamed" {
+		t.Fatalf("unexpected body: %q", string(body))
+	}
+	if got := resp.Trailer.Get("X-Unannounced-Trailer"); got != "later" {
+		t.Fatalf("unexpected unannounced trailer: %q", got)
+	}
+}
+
 func TestReverseProxyProtocolUpgrade(t *testing.T) {
 	t.Helper()