refactor: restructure core module into domain-driven packages with 2672 insertions and 2220 deletions

Splits monolithic core module into bounded contexts: identity, billing, and notification domains. Moves entity definitions, repository interfaces, and service implementations into separate packages. Updates import paths across 25 files to align with new package structure. Removes cross-domain coupling by extracting shared value objects into a common package.

.backup.1.nsock.c

@@ -0,0 +1,11 @@
+#include "nsock.h"
+
+void on_connect(int fd) { printf("connect\n"); }
+void on_data(int fd) { printf("data\n"); }
+void on_close(int fd) { printf("close\n"); }
+
+int main() {
+
+    nsock(9999, on_connect, on_data, on_close);
+    return 0;
+}