feat: add wrenEnsureSlots, wrenSetSlotNewList, and wrenInsertInList C API functions for list manipulation

Add three new public C API functions to the Wren embedding API that enable
foreign methods to create and populate lists. wrenEnsureSlots grows the
foreign slot stack to guarantee space for temporary work, wrenSetSlotNewList
stores a new empty list in a slot, and wrenInsertInList inserts a value from
one slot into a list at another slot at a given index (supporting negative
indices for relative positioning). The implementation includes a new internal
ensureStack helper that reallocates the fiber's stack and updates all
pointers (stack top, call frame starts, open upvalues, and foreign stack
start) when the reallocation moves the stack. New test files (lists.c,
lists.h, lists.wren) exercise creation, appending, insertion, and negative
index insertion, and the existing slots test is extended with an ensure test
that verifies slot count growth and slot usability.

test/api/lists.c

@@ -0,0 +1,46 @@
+#include <string.h>
+
+#include "lists.h"
+
+static void newList(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+}
+
+// Helper function to store a double in a slot then insert it into the list at
+// slot zero.
+static void insertNumber(WrenVM* vm, int index, double value)
+{
+  wrenSetSlotDouble(vm, 1, value);
+  wrenInsertInList(vm, 0, index, 1);
+}
+
+static void insert(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+  
+  wrenEnsureSlots(vm, 2);
+  
+  // Appending.
+  insertNumber(vm, 0, 1.0);
+  insertNumber(vm, 1, 2.0);
+  insertNumber(vm, 2, 3.0);
+  
+  // Inserting.
+  insertNumber(vm, 0, 4.0);
+  insertNumber(vm, 1, 5.0);
+  insertNumber(vm, 2, 6.0);
+  
+  // Negative indexes.
+  insertNumber(vm, -1, 7.0);
+  insertNumber(vm, -2, 8.0);
+  insertNumber(vm, -3, 9.0);
+}
+
+WrenForeignMethodFn listsBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static Lists.newList()") == 0) return newList;
+  if (strcmp(signature, "static Lists.insert()") == 0) return insert;
+
+  return NULL;
+}

test/api/lists.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn listsBindMethod(const char* signature);

test/api/lists.wren

@@ -0,0 +1,10 @@
+class Lists {
+  foreign static newList()
+  foreign static insert()
+}
+
+var list = Lists.newList()
+System.print(list is List) // expect: true
+System.print(list.count) // expect: 0
+
+System.print(Lists.insert()) // expect: [4, 5, 6, 1, 2, 3, 9, 8, 7]

test/api/main.c

@@ -7,6 +7,7 @@
 #include "benchmark.h"
 #include "call.h"
 #include "foreign_class.h"
+#include "lists.h"
 #include "slots.h"
 #include "value.h"
 
@@ -36,6 +37,9 @@ static WrenForeignMethodFn bindForeignMethod(
   method = foreignClassBindMethod(fullName);
   if (method != NULL) return method;
   
+  method = listsBindMethod(fullName);
+  if (method != NULL) return method;
+  
   method = slotsBindMethod(fullName);
   if (method != NULL) return method;
   

test/api/slots.c

@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include <string.h>
 
 #include "slots.h"
@@ -73,11 +74,38 @@ static void setSlots(WrenVM* vm)
   }
 }
 
+static void ensure(WrenVM* vm)
+{
+  int before = wrenGetSlotCount(vm);
+  
+  wrenEnsureSlots(vm, 20);
+  
+  int after = wrenGetSlotCount(vm);
+  
+  // Use the slots to make sure they're available.
+  for (int i = 0; i < 20; i++)
+  {
+    wrenSetSlotDouble(vm, i, i);
+  }
+  
+  int sum = 0;
+
+  for (int i = 0; i < 20; i++)
+  {
+    sum += (int)wrenGetSlotDouble(vm, i);
+  }
+  
+  char result[100];
+  sprintf(result, "%d -> %d (%d)", before, after, sum);
+  wrenSetSlotString(vm, 0, result);
+}
+
 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.ensure()") == 0) return ensure;
 
   return NULL;
 }

test/api/slots.wren

@@ -1,9 +1,8 @@
 class Slots {
   foreign static noSet
-
   foreign static getSlots(bool, num, string, bytes, value)
-
   foreign static setSlots(a, b, c, d)
+  foreign static ensure()
 }
 
 // If nothing is set in the return slot, it retains its previous value, the
@@ -14,3 +13,5 @@ var value = ["value"]
 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.ensure()) // expect: 1 -> 20 (190)