feat: add config_repl Phase 1 — HEAD-file storage, SHA-256, trim, stale check, sanitise_cn

- config_repl.h/.c: atomic versioned file save, pointer file, trim, stale check
- test_config_repl.c: 17 Unity tests covering all Phase 1 storage functions
- typemap.h: add repl/heartbeat types + union members for xce_request_t/xce_reply_t
- clusterconfig.h: add config_generation field to cluster_config_t (capnp schema ref)
- meson.build: add config_repl.c and test_config_repl.c to build targets
- controller_test.c: register test_config_repl dispatch function

src/include/clusterconfig.h

@@ -57,6 +57,7 @@ typedef struct {
     scaler_config_t**    hproviders;
     int                  nlimits;
     xce_limit_config_t** limits;
+    uint64_t             config_generation; /* monotonic audit counter from capnp schema */
 } cluster_config_t;
 
 #endif