feat: add explicit Stdout.flush() and remove automatic flush on System.write()

Remove the automatic fflush(stdout) call from the write() function in vm.c, which was causing gratuitous performance overhead on every System.write() invocation. Introduce a new Stdout class with a foreign static flush() method that delegates to fflush(stdout), giving users explicit control over when output is flushed. Update the io module registration in modules.c to include the Stdout class and bump MAX_CLASSES_PER_MODULE from 5 to 6. Add documentation for Stdout in doc/site/modules/io/stdout.markdown, update the io index and template to list the new class, and add flush() usage notes to the Stdin readByte() and readLine() docs. Implement the stdoutFlush() foreign function in io.c.

src/cli/modules.c

@@ -43,6 +43,7 @@ extern void stdinIsRawSet(WrenVM* vm);
 extern void stdinIsTerminal(WrenVM* vm);
 extern void stdinReadStart(WrenVM* vm);
 extern void stdinReadStop(WrenVM* vm);
+extern void stdoutFlush(WrenVM* vm);
 extern void schedulerCaptureMethods(WrenVM* vm);
 extern void timerStartTimer(WrenVM* vm);
 
@@ -61,7 +62,7 @@ extern void timerStartTimer(WrenVM* vm);
 // If you add a new class to the largest module below, make sure to bump this.
 // Note that it also includes an extra slot for the sentinel value indicating
 // the end of the list.
-#define MAX_CLASSES_PER_MODULE 5
+#define MAX_CLASSES_PER_MODULE 6
 
 // Describes one foreign method in a class.
 typedef struct
@@ -153,6 +154,9 @@ static ModuleRegistry modules[] =
       STATIC_METHOD("readStart_()", stdinReadStart)
       STATIC_METHOD("readStop_()", stdinReadStop)
     END_CLASS
+    CLASS(Stdout)
+      STATIC_METHOD("flush()", stdoutFlush)
+    END_CLASS
   END_MODULE
   MODULE(os)
     CLASS(Platform)

src/cli/vm.c

@@ -151,7 +151,6 @@ static WrenForeignClassMethods bindForeignClass(
 static void write(WrenVM* vm, const char* text)
 {
   printf("%s", text);
-  fflush(stdout);
 }
 
 static void reportError(WrenVM* vm, WrenErrorType type,