db_sync: skip SYNC_DONE in count>0 handler when received==0 — breaks bad_sig recv→send loop on client side

src/db_sync.c

@@ -1049,6 +1049,12 @@ static void db_handle_send_data(struct DB_SYNC_INSTANCE* si, uint64_t src, const
     }
 
     uint64_t lch8 = 0; if (mc > 0) db_chain_hash8_at(si, mc - 1, &lch8);
+    if (received == 0 && resp_count == 0) {
+        if (sp) { uint32_t nsp = mc > 0 ? mc - 1 : 0; if (sp->synced_pos < nsp) sp->synced_pos = nsp; sp->sync_state = 2; sp->sync_start_tb = 0; }
+        DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC, "sync [%s:%04llX] → SEND_DATA: 0 new records, skip SYNC_DONE (synced=%u)",
+                   SI_SHRT(si), (unsigned long long)(src >> 16), sp ? sp->synced_pos : 0);
+        return;
+    }
     uint8_t sd[13];
     sd[0] = DB_MSG_SYNC_DONE; memcpy(sd + 1, &mc, 4); memcpy(sd + 5, &lch8, 8);
     db_sync_send(si, src, sd, 13);