feat: add vfreeze/vthaw slots to intf_varray_t; designated initializers (P0-0b)

rtti header already present at rtti-0.0.2 with vfreeze_fn_t, vthaw_fn_t,
retain_fn_t, and the two new slots in intf_varray_t. No header modification
needed.

Convert intf_varray_t compound-literal initializers in zarray.c and kvmgr.c
to designated style with explicit .vfreeze = NULL and .vthaw = NULL slots.
kv.c has no intf_varray_t initializer.

Add test_zarray_cow.c with test_freeze_thaw_slots_exist which verifies the
struct layout has the slots accessible and that they are NULL until P0-2
wires the real implementations. Wire test into meson.build and test.c.

Full clean rebuild confirmed: 27/28 targets linked, no new test failures.
Only the 4 pre-existing failures remain.

src/lib/testsuite/test_zarray_cow.c

@@ -0,0 +1,35 @@
+#include <assert.h>
+#include <rtti.h>
+#include <stdio.h>
+#include <unity/unity.h>
+
+#include "zarray.h"
+
+/* xzarray_type is defined in zarray.c */
+extern typeinfo_t xzarray_type;
+
+static void test_freeze_thaw_slots_exist(void)
+{
+    /* After P0-0b, intf_varray_t has vfreeze and vthaw slots.
+     * They are NULL until P0-2 wires the implementations. */
+    const intf_varray_t* iface
+        = (const intf_varray_t*)xzarray_type.interfaces.varray;
+
+    TEST_ASSERT_NOT_NULL(iface);
+
+    /* Access slots without crash — verifies struct layout has them */
+    (void)iface->vfreeze;
+    (void)iface->vthaw;
+
+    /* Slots must be NULL until P0-2 wires the real implementations */
+    TEST_ASSERT_NULL(iface->vfreeze);
+    TEST_ASSERT_NULL(iface->vthaw);
+
+    printf("test_freeze_thaw_slots_exist: PASS\n");
+}
+
+void test_zarray_cow(void)
+{
+    printf("\n>> zarray COW interface slot test\n\n");
+    RUN_TEST(test_freeze_thaw_slots_exist);
+}