feat: prevent calling root fiber and refactor fiber state tracking

Add explicit check in `runFiber()` to reject calls to the root fiber, preventing VM crashes when a completed fiber's caller tries to resume it. Replace the boolean `callerIsTrying` field with a `FiberState` enum (`FIBER_TRY`, `FIBER_ROOT`, `FIBER_OTHER`) to track fiber invocation mode, updating all relevant primitives and initialization. Include new test cases (`call_root.wren`, `call_wren_call_root.*`) and re-entrancy documentation notes.

test/api/call_wren_call_root.c

@@ -0,0 +1,29 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "wren.h"
+#include "vm.h"
+
+void callWrenCallRootRunTests(WrenVM* vm)
+{
+  wrenEnsureSlots(vm, 1);
+  wrenGetVariable(vm, "./test/api/call_wren_call_root", "Test", 0);
+  WrenHandle* testClass = wrenGetSlotHandle(vm, 0);
+
+  WrenHandle* run = wrenMakeCallHandle(vm, "run()");
+
+  wrenEnsureSlots(vm, 1);
+  wrenSetSlotHandle(vm, 0, testClass);
+  WrenInterpretResult result = wrenCall(vm, run);
+  if (result == WREN_RESULT_RUNTIME_ERROR)
+  {
+    setExitCode(70);
+  }
+  else
+  {
+    printf("Missing runtime error.\n");
+  }
+
+  wrenReleaseHandle(vm, testClass);
+  wrenReleaseHandle(vm, run);
+}

test/api/call_wren_call_root.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+void callWrenCallRootRunTests(WrenVM* vm);

test/api/call_wren_call_root.wren

@@ -0,0 +1,12 @@
+class Test {
+  static run() {
+    var root = Fiber.current
+    System.print("begin root") // expect: begin root
+
+    Fiber.new {
+      System.print("in new fiber") // expect: in new fiber
+      root.call() // expect runtime error: Cannot call root fiber.
+      System.print("called root")
+    }.transfer()
+  }
+}

test/api/main.c

@@ -6,6 +6,7 @@
 
 #include "benchmark.h"
 #include "call.h"
+#include "call_wren_call_root.h"
 #include "error.h"
 #include "get_variable.h"
 #include "foreign_class.h"
@@ -101,6 +102,10 @@ static void afterLoad(WrenVM* vm)
   {
     callRunTests(vm);
   }
+  else if (strstr(testName, "/call_wren_call_root.wren") != NULL)
+  {
+    callWrenCallRootRunTests(vm);
+  }
   else if (strstr(testName, "/reset_stack_after_call_abort.wren") != NULL)
   {
     resetStackAfterCallAbortRunTests(vm);
@@ -122,5 +127,5 @@ int main(int argc, const char* argv[])
   testName = argv[1];
   setTestCallbacks(bindForeignMethod, bindForeignClass, afterLoad);
   runFile(testName);
-  return 0;
+  return getExitCode();
 }

test/core/fiber/call_root.wren

@@ -0,0 +1,8 @@
+var root = Fiber.current
+System.print("begin root") // expect: begin root
+
+Fiber.new {
+  System.print("in new fiber") // expect: in new fiber
+  root.call() // expect runtime error: Cannot call root fiber.
+  System.print("called root")
+}.transfer()