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/language/inheritance/inherit_from_class.wren

@@ -1 +1 @@
-class Subclass is Class {} // expect runtime error: Subclass cannot inherit from Class.
+class Subclass is Class {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Class'.

test/language/inheritance/inherit_from_closure.wren

@@ -5,4 +5,4 @@ var ClosureType
   ClosureType = Fn.new { IO.print(a) }.type
 }
 
-class Subclass is ClosureType {} // expect runtime error: Subclass cannot inherit from Fn.
+class Subclass is ClosureType {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fn'.

test/language/inheritance/inherit_from_fiber.wren

@@ -1 +1 @@
-class Subclass is Fiber {} // expect runtime error: Subclass cannot inherit from Fiber.
+class Subclass is Fiber {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fiber'.

test/language/inheritance/inherit_from_fn.wren

@@ -1 +1 @@
-class Subclass is Fn {} // expect runtime error: Subclass cannot inherit from Fn.
+class Subclass is Fn {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fn'.

test/language/inheritance/inherit_from_list.wren

@@ -1 +1 @@
-class Subclass is List {} // expect runtime error: Subclass cannot inherit from List.
+class Subclass is List {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'List'.

test/language/inheritance/inherit_from_map.wren

@@ -1 +1 @@
-class Subclass is Map {} // expect runtime error: Subclass cannot inherit from Map.
+class Subclass is Map {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Map'.

test/language/inheritance/inherit_from_nonclass.wren

@@ -1 +1 @@
-class Foo is 123 {} // expect runtime error: Must inherit from a class.
+class Foo is 123 {} // expect runtime error: Class 'Foo' cannot inherit from a non-class object.

test/language/inheritance/inherit_from_range.wren

@@ -1 +1 @@
-class Subclass is Range {} // expect runtime error: Subclass cannot inherit from Range.
+class Subclass is Range {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Range'.

test/language/inheritance/inherit_from_string.wren

@@ -1 +1 @@
-class Subclass is String {} // expect runtime error: Subclass cannot inherit from String.
+class Subclass is String {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'String'.

test/language/inheritance/inherit_methods.wren

@@ -22,5 +22,3 @@ bar.method(1, 2) // expect: bar
 bar.method(1, 2, 3) // expect: foo
 bar.method(1, 2, 3, 4) // expect: bar
 bar.override // expect: bar
-
-// TODO: Prevent extending built-in types.