feat: replace fiber run() with transfer() and suspend() for scheduler-friendly semantics

Remove the run() method from fibers and introduce transfer() for non-stack-based fiber switching, along with suspend() to pause the interpreter and return control to the host application. Update Timer.sleep() to use runNextScheduled_() instead of Fiber.yield(), and add Timer.schedule() for creating independently scheduled fibers. Refactor the core fiber runtime to support transfer semantics, including proper error handling for aborted fibers and self-transfer edge cases. Fix a missing quote in test.py's runtime error validation string.

src/module/timer.c

@@ -14,16 +14,36 @@ static const char* timerLibSource =
 "    if (!(milliseconds is Num)) Fiber.abort(\"Milliseconds must be a number.\")\n"
 "    if (milliseconds < 0) Fiber.abort(\"Milliseconds cannot be negative.\")\n"
 "    startTimer_(milliseconds, Fiber.current)\n"
-"    Fiber.yield()\n"
+"\n"
+"    runNextScheduled_()\n"
+"  }\n"
+"\n"
+"  // TODO: Once the CLI modules are more fleshed out, find a better place to\n"
+"  // put this.\n"
+"  static schedule(callable) {\n"
+"    if (__scheduled == null) __scheduled = []\n"
+"    __scheduled.add(Fiber.new {\n"
+"      callable.call()\n"
+"      runNextScheduled_()\n"
+"    })\n"
 "  }\n"
 "\n"
 "  foreign static startTimer_(milliseconds, fiber)\n"
 "\n"
 "  // Called by native code.\n"
 "  static resumeTimer_(fiber) {\n"
-"    fiber.run()\n"
+"    fiber.transfer()\n"
 "  }\n"
-"}\n";
+"\n"
+"  static runNextScheduled_() {\n"
+"    if (__scheduled == null || __scheduled.isEmpty) {\n"
+"      Fiber.suspend()\n"
+"    } else {\n"
+"      __scheduled.removeAt(0).transfer()\n"
+"    }\n"
+"  }\n"
+"}\n"
+"\n";
 
 // The Wren method to call when a timer has completed.
 static WrenMethod* resumeTimer;

src/module/timer.wren

@@ -3,13 +3,33 @@ class Timer {
     if (!(milliseconds is Num)) Fiber.abort("Milliseconds must be a number.")
     if (milliseconds < 0) Fiber.abort("Milliseconds cannot be negative.")
     startTimer_(milliseconds, Fiber.current)
-    Fiber.yield()
+
+    runNextScheduled_()
+  }
+
+  // TODO: Once the CLI modules are more fleshed out, find a better place to
+  // put this.
+  static schedule(callable) {
+    if (__scheduled == null) __scheduled = []
+    __scheduled.add(Fiber.new {
+      callable.call()
+      runNextScheduled_()
+    })
   }
 
   foreign static startTimer_(milliseconds, fiber)
 
   // Called by native code.
   static resumeTimer_(fiber) {
-    fiber.run()
+    fiber.transfer()
+  }
+
+  static runNextScheduled_() {
+    if (__scheduled == null || __scheduled.isEmpty) {
+      Fiber.suspend()
+    } else {
+      __scheduled.removeAt(0).transfer()
+    }
   }
 }
+