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.wren

@@ -0,0 +1,48 @@
+// Class with a default constructor.
+foreign class Counter {
+  foreign increment(amount)
+  foreign value
+}
+
+var counter = Counter.new()
+IO.print(counter.value) // expect: 0
+counter.increment(3.1)
+IO.print(counter.value) // expect: 3.1
+counter.increment(1.2)
+IO.print(counter.value) // expect: 4.3
+
+// Foreign classes can inherit a class as long as it has no fields.
+class PointBase {
+  inherited() {
+    IO.print("inherited method")
+  }
+}
+
+// Class with non-default constructor.
+foreign class Point is PointBase {
+  construct new() {
+    IO.print("default")
+  }
+
+  construct new(x, y, z) {
+    IO.print(x, ", ", y, ", ", z)
+  }
+
+  foreign translate(x, y, z)
+  foreign toString
+}
+
+var p = Point.new(1, 2, 3) // expect: 1, 2, 3
+IO.print(p) // expect: (1, 2, 3)
+p.translate(3, 4, 5)
+IO.print(p) // expect: (4, 6, 8)
+
+p = Point.new() // expect: default
+IO.print(p) // expect: (0, 0, 0)
+
+p.inherited() // expect: inherited method
+
+var error = Fiber.new {
+  class Subclass is Point {}
+}.try()
+IO.print(error) // expect: Class 'Subclass' cannot inherit from foreign class 'Point'.