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.

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)
-	w.res.raw(data.bytestr())
-	w.res.raw('\r\n')
+	// Write directly to socket fd
+	w.write_to_fd(chunk_header.bytes())!
+	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)
-	w.res.raw(data)
-	w.res.raw('\r\n')
+	// Write directly to socket fd
+	w.write_to_fd(chunk_header.bytes())!
+	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
-	// No explicit flush needed
+	// Direct fd writes are unbuffered, 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.res.end()
+		w.write_to_fd('0\r\n\r\n'.bytes()) or {}
 		w.connected = false
+		// Note: Don't close the fd here, picoev manages the connection lifecycle
 	}
 }