feat: add foreign class support with allocation, construct opcodes, and CLI callback wiring

Implement the initial infrastructure for foreign classes in the Wren VM, including new opcodes (FOREIGN_CONSTRUCT, FOREIGN_CLASS), a WrenBindForeignClassFn callback type in the public API, and updated CLI vm.c to store and forward foreign binding callbacks via setForeignCallbacks(). The compiler now tracks whether a class is foreign via a new isForeign flag in ClassCompiler, emits CODE_FOREIGN_CONSTRUCT instead of CODE_CONSTRUCT for foreign class constructors, and errors on field definitions inside foreign classes. The debug dump and opcode header gain entries for the new opcodes, and wrenBindSuperclass handles the -1 numFields sentinel for foreign classes to prevent field inheritance from superclasses.

test/api/foreign_class.c

@@ -0,0 +1,87 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "foreign_class.h"
+
+static void counterAllocate(WrenVM* vm)
+{
+  double* value = (double*)wrenAllocateForeign(vm, sizeof(double));
+  *value = 0;
+}
+
+static void counterIncrement(WrenVM* vm)
+{
+  double* value = (double*)wrenGetArgumentForeign(vm, 0);
+  double increment = wrenGetArgumentDouble(vm, 1);
+
+  *value += increment;
+}
+
+static void counterValue(WrenVM* vm)
+{
+  double value = *(double*)wrenGetArgumentForeign(vm, 0);
+  wrenReturnDouble(vm, value);
+}
+
+static void pointAllocate(WrenVM* vm)
+{
+  double* coordinates = (double*)wrenAllocateForeign(vm, sizeof(double[3]));
+
+  // This gets called by both constructors, so sniff the argument count to see
+  // which one was invoked.
+  if (wrenGetArgumentCount(vm) == 1)
+  {
+    coordinates[0] = 0.0;
+    coordinates[1] = 0.0;
+    coordinates[2] = 0.0;
+  }
+  else
+  {
+    coordinates[0] = wrenGetArgumentDouble(vm, 1);
+    coordinates[1] = wrenGetArgumentDouble(vm, 2);
+    coordinates[2] = wrenGetArgumentDouble(vm, 3);
+  }
+}
+
+static void pointTranslate(WrenVM* vm)
+{
+  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
+  coordinates[0] += wrenGetArgumentDouble(vm, 1);
+  coordinates[1] += wrenGetArgumentDouble(vm, 2);
+  coordinates[2] += wrenGetArgumentDouble(vm, 3);
+}
+
+static void pointToString(WrenVM* vm)
+{
+  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
+  char result[100];
+  sprintf(result, "(%g, %g, %g)",
+      coordinates[0], coordinates[1], coordinates[2]);
+  wrenReturnString(vm, result, (int)strlen(result));
+}
+
+WrenForeignMethodFn foreignClassBindMethod(const char* signature)
+{
+  if (strcmp(signature, "Counter.increment(_)") == 0) return counterIncrement;
+  if (strcmp(signature, "Counter.value") == 0) return counterValue;
+  if (strcmp(signature, "Point.translate(_,_,_)") == 0) return pointTranslate;
+  if (strcmp(signature, "Point.toString") == 0) return pointToString;
+
+  return NULL;
+}
+
+void foreignClassBindClass(
+    const char* className, WrenForeignClassMethods* methods)
+{
+  if (strcmp(className, "Counter") == 0)
+  {
+    methods->allocate = counterAllocate;
+    return;
+  }
+
+  if (strcmp(className, "Point") == 0)
+  {
+    methods->allocate = pointAllocate;
+    return;
+  }
+}