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.

src/vm/wren_core.c

@@ -88,8 +88,14 @@ static bool runFiber(WrenVM* vm, ObjFiber* fiber, Value* args, bool isCall,
 
   if (isCall)
   {
+    // You can't call a called fiber, but you can transfer directly to it,
+    // which is why this check is gated on `isCall`. This way, after resuming a
+    // suspended fiber, it will run and then return to the fiber that called it
+    // and so on.
     if (fiber->caller != NULL) RETURN_ERROR("Fiber has already been called.");
 
+    if (fiber->state == FIBER_ROOT) RETURN_ERROR("Cannot call root fiber.");
+    
     // Remember who ran it.
     fiber->caller = vm->fiber;
   }
@@ -181,7 +187,7 @@ DEF_PRIMITIVE(fiber_try)
   runFiber(vm, AS_FIBER(args[0]), args, true, false, "try");
   
   // If we're switching to a valid fiber to try, remember that we're trying it.
-  if (IS_NULL(vm->fiber->error)) vm->fiber->callerIsTrying = true;
+  if (IS_NULL(vm->fiber->error)) vm->fiber->state = FIBER_TRY;
   return false;
 }
 
@@ -192,7 +198,7 @@ DEF_PRIMITIVE(fiber_yield)
 
   // Unhook this fiber from the one that called it.
   current->caller = NULL;
-  current->callerIsTrying = false;
+  current->state = FIBER_OTHER;
 
   if (vm->fiber != NULL)
   {
@@ -210,7 +216,7 @@ DEF_PRIMITIVE(fiber_yield1)
 
   // Unhook this fiber from the one that called it.
   current->caller = NULL;
-  current->callerIsTrying = false;
+  current->state = FIBER_OTHER;
 
   if (vm->fiber != NULL)
   {

src/vm/wren_value.c

@@ -171,7 +171,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, ObjClosure* closure)
   fiber->openUpvalues = NULL;
   fiber->caller = NULL;
   fiber->error = NULL_VAL;
-  fiber->callerIsTrying = false;
+  fiber->state = FIBER_OTHER;
   
   if (closure != NULL)
   {

src/vm/wren_value.h

@@ -293,6 +293,24 @@ typedef struct
   Value* stackStart;
 } CallFrame;
 
+// Tracks how this fiber has been invoked, aside from the ways that can be
+// detected from the state of other fields in the fiber.
+typedef enum
+{
+  // The fiber is being run from another fiber using a call to `try()`.
+  FIBER_TRY,
+  
+  // The fiber was directly invoked by `runInterpreter()`. This means it's the
+  // initial fiber used by a call to `wrenCall()` or `wrenInterpret()`.
+  FIBER_ROOT,
+  
+  // The fiber is invoked some other way. If [caller] is `NULL` then the fiber
+  // was invoked using `call()`. If [numFrames] is zero, then the fiber has
+  // finished running and is done. If [numFrames] is one and that frame's `ip`
+  // points to the first byte of code, the fiber has not been started yet.
+  FIBER_OTHER,
+} FiberState;
+
 typedef struct sObjFiber
 {
   Obj obj;
@@ -331,10 +349,7 @@ typedef struct sObjFiber
   // error object. Otherwise, it will be null.
   Value error;
   
-  // This will be true if the caller that called this fiber did so using "try".
-  // In that case, if this fiber fails with an error, the error will be given
-  // to the caller.
-  bool callerIsTrying;
+  FiberState state;
 } ObjFiber;
 
 typedef enum

src/vm/wren_vm.c

@@ -399,7 +399,7 @@ static void runtimeError(WrenVM* vm)
     current->error = error;
 
     // If the caller ran this fiber using "try", give it the error and stop.
-    if (current->callerIsTrying)
+    if (current->state == FIBER_TRY)
     {
       // Make the caller's try method return the error message.
       current->caller->stackTop[-1] = vm->fiber->error;
@@ -768,12 +768,12 @@ static Value getModuleVariable(WrenVM* vm, ObjModule* module,
 }
 
 // The main bytecode interpreter loop. This is where the magic happens. It is
-// also, as you can imagine, highly performance critical. Returns `true` if the
-// fiber completed without error.
+// also, as you can imagine, highly performance critical.
 static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
 {
   // Remember the current fiber so we can find it if a GC happens.
   vm->fiber = fiber;
+  fiber->state = FIBER_ROOT;
 
   // Hoist these into local variables. They are accessed frequently in the loop
   // but assigned less frequently. Keeping them in locals and updating them when