fix: assign reallocation result to list->elements in shrink path

When shrinking a list's capacity, wrenReallocate may return a different
memory address. The previous code ignored the return value, leaving
list->elements pointing to freed memory. This fix assigns the result
back to list->elements and adds a regression test for grow/shrink
behavior.

test/list/grow_shrink.wren

@@ -0,0 +1,11 @@
+// This mostly tests that lists handle growing and shrinking their memory.
+var list = []
+for (i in 0..200) {
+  list.add(i)
+}
+
+for (i in 0..195) {
+  list.removeAt(-1)
+}
+
+IO.print(list) // expect: [0, 1, 2, 3, 4]