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/testsuite/test.c

@@ -30,6 +30,7 @@ extern void test_qobj(void);
 extern void test_wobj(void);
 extern void test_request(void);
 extern void test_zarray_cow(void);
+extern void test_kvmgr_capture(void);
 
 void setUp(void) { return; }
 
@@ -129,6 +130,7 @@ int main(int argc, char* argv[])
     TEST(test_qobj);
     TEST(test_request);
     TEST(test_zarray_cow);
+    TEST(test_kvmgr_capture);
 
     return UNITY_END();
 }