feat: expose wrenEnsureSlots API and rename foreignStackStart to apiStack for slot access outside foreign methods

Replace the internal `foreignStackStart` pointer with a public `apiStack` field
that tracks the base of available C API slots. Extract the static
`ensureStack` helper into a non-static `wrenEnsureStack` function declared in
the header, enabling `wrenEnsureSlots()` to create a fiber and stack when
called outside a foreign method. Update `callForeign` to use `apiStack` and
adjust `metaCompile` to write results through the new pointer. Add a
`ensureOutsideForeign` test that allocates slots on a fresh VM, verifies
slot count grows from 0 to 20, and exercises all 20 slots with double values
to confirm the stack is correctly initialized and usable.

test/api/slots.c

@@ -94,7 +94,41 @@ static void ensure(WrenVM* vm)
   {
     sum += (int)wrenGetSlotDouble(vm, i);
   }
-  
+
+  char result[100];
+  sprintf(result, "%d -> %d (%d)", before, after, sum);
+  wrenSetSlotString(vm, 0, result);
+}
+
+static void ensureOutsideForeign(WrenVM* vm)
+{
+  // To test the behavior outside of a foreign method (which we're currently
+  // in), create a new separate VM.
+  WrenConfiguration config;
+  wrenInitConfiguration(&config);
+  WrenVM* otherVM = wrenNewVM(&config);
+
+  int before = wrenGetSlotCount(otherVM);
+
+  wrenEnsureSlots(otherVM, 20);
+
+  int after = wrenGetSlotCount(otherVM);
+
+  // Use the slots to make sure they're available.
+  for (int i = 0; i < 20; i++)
+  {
+    wrenSetSlotDouble(otherVM, i, i);
+  }
+
+  int sum = 0;
+
+  for (int i = 0; i < 20; i++)
+  {
+    sum += (int)wrenGetSlotDouble(otherVM, i);
+  }
+
+  wrenFreeVM(otherVM);
+
   char result[100];
   sprintf(result, "%d -> %d (%d)", before, after, sum);
   wrenSetSlotString(vm, 0, result);
@@ -106,6 +140,7 @@ WrenForeignMethodFn slotsBindMethod(const char* signature)
   if (strcmp(signature, "static Slots.getSlots(_,_,_,_,_)") == 0) return getSlots;
   if (strcmp(signature, "static Slots.setSlots(_,_,_,_)") == 0) return setSlots;
   if (strcmp(signature, "static Slots.ensure()") == 0) return ensure;
+  if (strcmp(signature, "static Slots.ensureOutsideForeign()") == 0) return ensureOutsideForeign;
 
   return NULL;
 }

test/api/slots.wren

@@ -3,6 +3,7 @@ class Slots {
   foreign static getSlots(bool, num, string, bytes, value)
   foreign static setSlots(a, b, c, d)
   foreign static ensure()
+  foreign static ensureOutsideForeign()
 }
 
 // If nothing is set in the return slot, it retains its previous value, the
@@ -10,8 +11,14 @@ class Slots {
 System.print(Slots.noSet == Slots) // expect: true
 
 var value = ["value"]
-System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value) // expect: true
+System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value)
+// expect: true
 
-System.print(Slots.setSlots(value, 0, 0, 0) == value) // expect: true
+System.print(Slots.setSlots(value, 0, 0, 0) == value)
+// expect: true
 
-System.print(Slots.ensure()) // expect: 1 -> 20 (190)
+System.print(Slots.ensure())
+// expect: 1 -> 20 (190)
+
+System.print(Slots.ensureOutsideForeign())
+// expect: 0 -> 20 (190)