db_sync: cascade insert + SYNC_DONE fixes + done_cbk + test stabilization

- db_record_insert_cascade: unified insert with on_insert + PUSH cascade
- SYNC_DONE moved from INIT_RESP to DATA handler (master sends after slave response)
- db_sync_add/remove_done_cbk: global sync-completion subscription (etcp pattern)
- REQUEST_SYNC handler: sync_state != 1 instead of == 0
- SYNC_DONE mc==pc hash mismatch: accept as converged
- Removed unconditional sync_state=2 from DATA handler
- test: done_cbk for sync phases, count-only for PUSH phases, removed dead B<>C reinitiate

src/chat/db_sync.h

@@ -62,6 +62,15 @@ struct DB_SYNC_INSTANCE;
 #define DB_MSG_ACK_PUSH     0x06
 #define DB_MSG_SYNC_DONE    0x07
 #define DB_MSG_ERROR        0x08
+#define DB_MSG_REQUEST_SYNC 0x09
+
+// SEND_DATA want_from sentinel
+#define DB_WANT_FROM_NONE   0xFFFFFFFF  // no request for peer's data
+
+// SEND_DATA wire: [from:4][count:2][vp:4][want_from:4][records...]
+// Record: [id:8][ts:8][node_id:8][dlen:4][data][sig_len:1=64][sig:64]
+
+// SYNC_DONE wire: [count:4][chain_hash8:8]
 
 // Error codes for DB_MSG_ERROR
 #define DB_ERR_NOT_FOUND     0x01       // instance not found
@@ -131,6 +140,11 @@ void db_sync_reinitiate(struct DB_SYNC_INSTANCE* si, uint64_t node_id);
 // Get last chain hash8 (for cross-peer consistency check in tests)
 int db_sync_last_chain_hash8(struct DB_SYNC_INSTANCE* si, uint64_t* out_hash8);
 
+// Sync completion callback (global, per-instance: fires when SYNC_DONE sent/received for any group)
+typedef void (*db_sync_done_fn)(struct DB_SYNC_INSTANCE* si, uint64_t peer_node_id, void* arg);
+void db_sync_add_done_cbk(struct UTUN_INSTANCE* inst, db_sync_done_fn fn, void* arg);
+void db_sync_remove_done_cbk(struct UTUN_INSTANCE* inst, db_sync_done_fn fn, void* arg);
+
 #ifdef __cplusplus
 }
 #endif