mruby-io: implement << operator in c for improved performance

Moved IO#<< from Ruby to C implementation to reduce boundary
crossing overhead. Maintains full compatibility with automatic
to_s conversion and proper return value for method chaining.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-io/mrblib/io.rb

@@ -157,20 +157,6 @@ class IO
     self.__id__
   end
 
-  #
-  # call-seq:
-  #   ios << obj     -> ios
-  #
-  # String Output - Writes obj to ios. obj will be converted to a string using
-  # to_s.
-  #
-  #   $stdout << "Hello " << "world!\n"
-  #   Hello world!
-  #
-  def <<(str)
-    write(str)
-    self
-  end
 
   # Alias for eof?
   alias_method :eof, :eof?

mrbgems/mruby-io/src/io.c

@@ -1248,6 +1248,29 @@ io_print(mrb_state *mrb, mrb_value io)
   return mrb_nil_value();
 }
 
+/*
+ * call-seq:
+ *   ios << obj     -> ios
+ *
+ * String Output - Writes obj to ios. obj will be converted to a string using
+ * to_s.
+ */
+static mrb_value
+io_lshift(mrb_state *mrb, mrb_value io)
+{
+  struct mrb_io *fptr = io_get_write_fptr(mrb, io);
+  int fd = io_get_write_fd(fptr);
+
+  /* Prepare IO for writing (handle read buffer adjustment) */
+  io_prepare_write(mrb, fptr);
+
+  mrb_value str = mrb_get_arg1(mrb);
+  str = mrb_obj_as_string(mrb, str);
+  fd_write(mrb, fd, str);
+
+  return io;
+}
+
 static mrb_value
 io_close(mrb_state *mrb, mrb_value io)
 {
@@ -2257,6 +2280,7 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method_id(mrb, io, MRB_SYM(write),      io_write,      MRB_ARGS_ANY());    /* 15.2.20.5.20 */
   mrb_define_method_id(mrb, io, MRB_SYM(puts),       io_puts,       MRB_ARGS_ANY());
   mrb_define_method_id(mrb, io, MRB_SYM(print),      io_print,      MRB_ARGS_ANY());
+  mrb_define_method_id(mrb, io, MRB_OPSYM(lshift),   io_lshift,     MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, io, MRB_SYM(pread),      io_pread,      MRB_ARGS_ANY());    /* Ruby 2.5 feature */
   mrb_define_method_id(mrb, io, MRB_SYM(pwrite),     io_pwrite,     MRB_ARGS_ANY());    /* Ruby 2.5 feature */
   mrb_define_method_id(mrb, io, MRB_SYM(getbyte),    io_getbyte,    MRB_ARGS_NONE());