refactor: remove oldSize parameter from WrenReallocateFn and all call sites

Simplify the allocation API by dropping the unused `oldSize` argument from `WrenReallocateFn` typedef, its documentation, and all internal callers. Update `wrenReallocate` in `wren_vm.c` to pass only `newSize`, remove the `defaultReallocate` wrapper in favor of direct `realloc`, and adjust `generate_baseline` signature in benchmark script to match.

src/include/wren.h

@@ -14,20 +14,19 @@ typedef struct WrenMethod WrenMethod;
 // A generic allocation function that handles all explicit memory management
 // used by Wren. It's used like so:
 //
-// - To allocate new memory, [memory] is NULL and [oldSize] is zero. It should
-//   return the allocated memory or NULL on failure.
+// - To allocate new memory, [memory] is NULL and [newSize] is the desired
+//   size. It should return the allocated memory or NULL on failure.
 //
-// - To attempt to grow an existing allocation, [memory] is the memory,
-//   [oldSize] is its previous size, and [newSize] is the desired size.
-//   It should return [memory] if it was able to grow it in place, or a new
-//   pointer if it had to move it.
+// - To attempt to grow an existing allocation, [memory] is the memory, and
+//   [newSize] is the desired size. It should return [memory] if it was able to
+//   grow it in place, or a new pointer if it had to move it.
 //
-// - To shrink memory, [memory], [oldSize], and [newSize] are the same as above
-//   but it will always return [memory].
+// - To shrink memory, [memory] and [newSize] are the same as above but it will
+//   always return [memory].
 //
-// - To free memory, [memory] will be the memory to free and [newSize] and
-//   [oldSize] will be zero. It should return NULL.
-typedef void* (*WrenReallocateFn)(void* memory, size_t oldSize, size_t newSize);
+// - To free memory, [memory] will be the memory to free and [newSize] will be
+//   zero. It should return NULL.
+typedef void* (*WrenReallocateFn)(void* memory, size_t newSize);
 
 // A function callable from Wren code, but implemented in C.
 typedef void (*WrenForeignMethodFn)(WrenVM* vm);