feat: implement kvmgr_capture_keys for lock-free checkpoint serialization (P0-14a)

Add kvtab_foreach walk primitive (P0-14) to kv.c/kv.h, then implement
kvmgr_capture_keys in kvmgr.c that deep-copies all key→index mappings
into a malloc'd slot_key_t[] array safe to read after the write lock is
released during checkpoint serialization (Part 3.8).

Key design points:
- Both kvtab_foreach and kvmgr_capture_keys take var (RTTI data pointer)
  and use RTTI_DATA_TO_OBJ internally — the RTTI framework's makeInstance
  returns the data region pointer, not the meta_obj_t header pointer
- String keys are copied via strncpy into slot_key_t.key[XCE_MAXNAME_LEN]
- Integer keys are formatted as decimal strings via snprintf
- Two-pass approach: count live entries first, allocate, then fill
- Tests cover string kvmgr (3 entries), empty kvmgr, and deep-copy safety

src/lib/meson.build

@@ -164,6 +164,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_qobj.c',
                    'testsuite/test_request.c',
                    'testsuite/test_zarray_cow.c',
+                   'testsuite/test_kvmgr_capture.c',
                   ]
 
 libxce_test_deps = [libxce_deps, librtti_dep, unity_dep, libxce_dep]