fix(streaming): use direct socket fd writes for real-time SSE streaming

Previously, PicoevStreamWriter used picohttpparser.Response.raw() to write
streaming data, which only buffered the data until the entire request handler
completed. This caused SSE responses to be delayed until all sleep() calls
finished (e.g., 2.89s delay for /stream-text endpoint).

Changes:
- Modified PicoevStreamWriter to use socket fd directly from picohttpparser.Response
- Implemented write_to_fd() using C.send() for immediate data delivery
- Updated handle_picoev_streaming_response() to write HTTP headers directly to socket
- Added C.send() function declaration for direct socket writes

This fix enables true real-time streaming where each write() or write_sse()
call immediately sends data to the client, rather than buffering until
the callback completes.

All 74 test files pass with no regressions.

picoev_server.v

@@ -583,8 +583,8 @@ fn send_picoev_response(mut res picohttpparser.Response, ctx Context, response h
 
 // handle_picoev_streaming_response - Handle a streaming response using PicoevStreamWriter
 // This function:
-// 1. Sends the HTTP headers for streaming
+// 1. Sends the HTTP headers for streaming directly to socket fd
-// 2. Creates a PicoevStreamWriter
+// 2. Creates a PicoevStreamWriter with the socket fd
 // 3. Executes the streaming callback
 // 4. Handles errors and cleanup
 fn handle_picoev_streaming_response(mut res picohttpparser.Response, ctx Context, config PicoevConfig) {
@@ -601,14 +601,10 @@ fn handle_picoev_streaming_response(mut res picohttpparser.Response, ctx Context
 	// Determine status code
 	status_code := if ctx.status_code != 0 { ctx.status_code } else { 200 }
 	
-	// Send HTTP response line
+	// Build HTTP headers string for direct socket write
-	if status_code == 200 {
+	mut headers_str := 'HTTP/1.1 ${status_code} ${get_status_text(status_code)}\r\n'
-		res.http_ok()
-	} else {
-		res.raw('HTTP/1.1 ${status_code} ${get_status_text(status_code)}\r\n')
-	}
 	
-	// Send headers from context
+	// Add headers from context
 	mut has_transfer_encoding := false
 	mut has_connection := false
 	
@@ -618,7 +614,7 @@ fn handle_picoev_streaming_response(mut res picohttpparser.Response, ctx Context
 		if key_len == 14 && eq_ignore_case(key, 'content-length') {
 			continue
 		}
-		res.header(key, value)
+		headers_str += '${key}: ${value}\r\n'
 		if key_len == 17 && eq_ignore_case(key, 'transfer-encoding') {
 			has_transfer_encoding = true
 		} else if key_len == 10 && eq_ignore_case(key, 'connection') {
@@ -628,20 +624,27 @@ fn handle_picoev_streaming_response(mut res picohttpparser.Response, ctx Context
 	
 	// Ensure Transfer-Encoding: chunked is set
 	if !has_transfer_encoding {
-		res.header('Transfer-Encoding', 'chunked')
+		headers_str += 'Transfer-Encoding: chunked\r\n'
 	}
 	
 	// Ensure Connection header is set for streaming
 	if !has_connection {
-		res.header('Connection', 'keep-alive')
+		headers_str += 'Connection: keep-alive\r\n'
-		res.header('Keep-Alive', 'timeout=${config.timeout_secs}')
+		headers_str += 'Keep-Alive: timeout=${config.timeout_secs}\r\n'
 	}
 	
-	// End headers section (empty body to signal headers are complete)
+	// End headers section
-	// Note: We don't call res.body() or res.end() here because we're streaming
+	headers_str += '\r\n'
-	res.raw('\r\n')
 	
-	// Create PicoevStreamWriter for streaming
+	// Write headers directly to socket fd for immediate delivery
+	fd := res.fd
+	write_to_socket_fd(fd, headers_str.bytes()) or {
+		eprintln('[Picoev Stream Error] Failed to write headers: ${err.msg()}')
+		cleanup_stream_config(ctx)
+		return
+	}
+	
+	// Create PicoevStreamWriter for streaming (uses fd directly)
 	mut writer := PicoevStreamWriter.new(res)
 	
 	// Create StreamContext with the writer
@@ -665,6 +668,27 @@ fn handle_picoev_streaming_response(mut res picohttpparser.Response, ctx Context
 	cleanup_stream_config(ctx)
 }
 
+// write_to_socket_fd - Helper function to write bytes directly to socket fd
+// Used for sending HTTP headers before streaming begins
+fn write_to_socket_fd(fd int, data []u8) ! {
+	if data.len == 0 {
+		return
+	}
+	mut total_written := 0
+	for total_written < data.len {
+		remaining := data.len - total_written
+		ptr := unsafe { &u8(data.data) + total_written }
+		written := C.send(fd, ptr, remaining, 0)
+		if written < 0 {
+			return error('Failed to write to socket')
+		}
+		if written == 0 {
+			return error('Connection closed by peer')
+		}
+		total_written += written
+	}
+}
+
 // 获取状态码文本
 fn get_status_text(code int) string {
 	return match code {

streaming.v

@@ -8,6 +8,9 @@ import time
 import picohttpparser
 import usockets
 
+// C function declaration for direct socket writes
+fn C.send(sockfd int, buf voidptr, len int, flags int) int
+
 // ============================================================================
 // SSE Event Structure
 // ============================================================================
@@ -184,23 +187,26 @@ pub fn (mut ctx StreamContext) trigger_abort() {
 
 // PicoevStreamWriter - StreamWriter implementation for picoev server
 // Supports chunked transfer encoding for streaming responses
+// Uses direct socket fd writes for immediate data delivery (true streaming)
 @[heap]
 pub struct PicoevStreamWriter {
 mut:
-	res       &picohttpparser.Response
+	fd        int  // Socket file descriptor for direct writes
 	connected bool
 }
 
 // PicoevStreamWriter.new - Create a new PicoevStreamWriter with the given response
+// Extracts the fd from picohttpparser.Response for direct socket writes
 pub fn PicoevStreamWriter.new(res &picohttpparser.Response) &PicoevStreamWriter {
 	return &PicoevStreamWriter{
-		res: unsafe { res }
+		fd: res.fd
 		connected: true
 	}
 }
 
 // write - Write raw bytes to the stream using chunked transfer encoding
 // Format: {hex_size}\r\n{data}\r\n
+// Writes directly to socket fd for immediate delivery
 pub fn (mut w PicoevStreamWriter) write(data []u8) ! {
 	if !w.connected {
 		return error('Connection closed')
@@ -210,9 +216,10 @@ pub fn (mut w PicoevStreamWriter) write(data []u8) ! {
 	}
 	// Write chunked format: size in hex, CRLF, data, CRLF
 	chunk_header := format_chunk_size(data.len)
-	w.res.raw(chunk_header)
+	// Write directly to socket fd
-	w.res.raw(data.bytestr())
+	w.write_to_fd(chunk_header.bytes())!
-	w.res.raw('\r\n')
+	w.write_to_fd(data)!
+	w.write_to_fd('\r\n'.bytes())!
 }
 
 // write_string - Write a string to the stream using chunked transfer encoding
@@ -225,15 +232,40 @@ pub fn (mut w PicoevStreamWriter) write_string(data string) ! {
 	}
 	// Write chunked format: size in hex, CRLF, data, CRLF
 	chunk_header := format_chunk_size(data.len)
-	w.res.raw(chunk_header)
+	// Write directly to socket fd
-	w.res.raw(data)
+	w.write_to_fd(chunk_header.bytes())!
-	w.res.raw('\r\n')
+	w.write_to_fd(data.bytes())!
+	w.write_to_fd('\r\n'.bytes())!
 }
 
-// flush - Flush the buffer (picoev handles this automatically)
+// write_to_fd - Write bytes directly to the socket file descriptor
+// This ensures immediate delivery without buffering
+@[inline]
+fn (mut w PicoevStreamWriter) write_to_fd(data []u8) ! {
+	if data.len == 0 {
+		return
+	}
+	mut total_written := 0
+	for total_written < data.len {
+		remaining := data.len - total_written
+		ptr := unsafe { &u8(data.data) + total_written }
+		written := C.send(w.fd, ptr, remaining, 0)
+		if written < 0 {
+			w.connected = false
+			return error('Failed to write to socket')
+		}
+		if written == 0 {
+			w.connected = false
+			return error('Connection closed by peer')
+		}
+		total_written += written
+	}
+}
+
+// flush - Flush the buffer (direct fd writes are already unbuffered)
 pub fn (mut w PicoevStreamWriter) flush() ! {
-	// picoev automatically flushes data
+	// Direct fd writes are unbuffered, no explicit flush needed
-	// No explicit flush needed
+	// But we can use TCP_NODELAY to disable Nagle's algorithm if needed
 }
 
 // close - Close the stream by sending the final chunk
@@ -241,9 +273,9 @@ pub fn (mut w PicoevStreamWriter) flush() ! {
 pub fn (mut w PicoevStreamWriter) close() ! {
 	if w.connected {
 		// Write the final chunk (zero-length chunk) to signal end of stream
-		w.res.raw('0\r\n\r\n')
+		w.write_to_fd('0\r\n\r\n'.bytes()) or {}
-		w.res.end()
 		w.connected = false
+		// Note: Don't close the fd here, picoev manages the connection lifecycle
 	}
 }