feat: add wrenGetSlotType API to query low-level type of object in a slot

Introduce a new public C API function wrenGetSlotType() that returns the
underlying representation type (bool, num, foreign, list, null, string,
or unknown) of the object stored in a given VM slot. This allows C
extensions to inspect slot contents without needing to call individual
type-checking functions or handle errors from mismatched accessors.

The implementation in wren_vm.c uses the existing IS_* macros to
determine the type and returns the corresponding WrenType enum value.
A new WrenType enum is added to wren.h with WREN_TYPE_BOOL,
WREN_TYPE_NUM, WREN_TYPE_FOREIGN, WREN_TYPE_LIST, WREN_TYPE_NULL,
WREN_TYPE_STRING, and WREN_TYPE_UNKNOWN.

Test coverage is added in test/api/slots.c with a slotTypes() foreign
method that verifies all seven type values, and a ForeignType foreign
class is introduced in slots.wren to provide a foreign object for
testing WREN_TYPE_FOREIGN. The test harness in main.c is updated to
support binding foreign class allocate methods via slotsBindClass().

test/api/main.c

@@ -63,7 +63,15 @@ static WrenForeignClassMethods bindForeignClass(
   if (strcmp(module, "main") != 0) return methods;
 
   foreignClassBindClass(className, &methods);
-
+  if (methods.allocate != NULL) return methods;
+  
+  slotsBindClass(className, &methods);
+  if (methods.allocate != NULL) return methods;
+  
+  
+  fprintf(stderr,
+          "Unknown foreign class '%s' for test '%s'\n", className, testName);
+  exit(1);
   return methods;
 }
 

test/api/slots.c

@@ -74,6 +74,20 @@ static void setSlots(WrenVM* vm)
   }
 }
 
+static void slotTypes(WrenVM* vm)
+{
+  bool result =
+      wrenGetSlotType(vm, 1) == WREN_TYPE_BOOL &&
+      wrenGetSlotType(vm, 2) == WREN_TYPE_FOREIGN &&
+      wrenGetSlotType(vm, 3) == WREN_TYPE_LIST &&
+      wrenGetSlotType(vm, 4) == WREN_TYPE_NULL &&
+      wrenGetSlotType(vm, 5) == WREN_TYPE_NUM &&
+      wrenGetSlotType(vm, 6) == WREN_TYPE_STRING &&
+      wrenGetSlotType(vm, 7) == WREN_TYPE_UNKNOWN;
+  
+  wrenSetSlotBool(vm, 0, result);
+}
+
 static void ensure(WrenVM* vm)
 {
   int before = wrenGetSlotCount(vm);
@@ -134,13 +148,24 @@ static void ensureOutsideForeign(WrenVM* vm)
   wrenSetSlotString(vm, 0, result);
 }
 
+static void foreignClassAllocate(WrenVM* vm)
+{
+  wrenSetSlotNewForeign(vm, 0, 0, 4);
+}
+
 WrenForeignMethodFn slotsBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Slots.noSet") == 0) return noSet;
   if (strcmp(signature, "static Slots.getSlots(_,_,_,_,_)") == 0) return getSlots;
   if (strcmp(signature, "static Slots.setSlots(_,_,_,_)") == 0) return setSlots;
+  if (strcmp(signature, "static Slots.slotTypes(_,_,_,_,_,_,_)") == 0) return slotTypes;
   if (strcmp(signature, "static Slots.ensure()") == 0) return ensure;
   if (strcmp(signature, "static Slots.ensureOutsideForeign()") == 0) return ensureOutsideForeign;
 
   return NULL;
 }
+
+void slotsBindClass(const char* className, WrenForeignClassMethods* methods)
+{
+  methods->allocate = foreignClassAllocate;
+}

test/api/slots.h

@@ -1,3 +1,4 @@
 #include "wren.h"
 
 WrenForeignMethodFn slotsBindMethod(const char* signature);
+void slotsBindClass(const char* className, WrenForeignClassMethods* methods);

test/api/slots.wren

@@ -2,10 +2,15 @@ class Slots {
   foreign static noSet
   foreign static getSlots(bool, num, string, bytes, value)
   foreign static setSlots(a, b, c, d)
+  foreign static slotTypes(bool, foreignObj, list, nullObj, num, string, unknown)
   foreign static ensure()
   foreign static ensureOutsideForeign()
 }
 
+foreign class ForeignType {
+  construct new() {}
+}
+
 // If nothing is set in the return slot, it retains its previous value, the
 // receiver.
 System.print(Slots.noSet == Slots) // expect: true
@@ -17,6 +22,9 @@ System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value)
 System.print(Slots.setSlots(value, 0, 0, 0) == value)
 // expect: true
 
+System.print(Slots.slotTypes(false, ForeignType.new(), [], null, 1.2, "str", 1..2))
+// expect: true
+
 System.print(Slots.ensure())
 // expect: 1 -> 20 (190)