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.rnet.c

@@ -0,0 +1,42 @@
+#include "rnet.h"
+
+void on_client_connect(rnet_socket_t *sock) { printf("%s connected\n", sock->name); }
+void on_client_read(rnet_socket_t *sock) {
+    unsigned char *data = net_socket_read(sock, 4096);
+    if (!data)
+        return;
+    char *http_headers = "HTTP/1.1 200 OK\r\nContent-Length: 10\r\nConnection: close\r\n\r\n";
+    net_socket_write(sock, (unsigned char *)http_headers, strlen(http_headers));
+    rnet_safe_str((char *)data, sock->bytes_received);
+    // data[11] = 0;
+    printf("%s: %.30s\n", sock->name, data);
+    net_socket_write(sock, data, strlen((char *)data));
+    if (!strncmp((char *)data, "GET ", 4))
+        net_socket_close(sock);
+}
+void on_client_close(rnet_socket_t *sock) { printf("%s disconnected\n", sock->name); }
+
+int main(int argc, char *argv[]) {
+    if (argc < 2) {
+        printf("usage: [port].\n");
+        return 1;
+    }
+    for (int i = 0; i < argc; i++) {
+        if (strcmp(argv[i], "test") == 0) {
+            printf("Skipping rnet tests.\n");
+            return 0;
+        }
+    }
+    rnet_server_t *server = net_socket_serve((unsigned int)atoi(argv[1]), 10);
+    server->on_connect = on_client_connect;
+    server->on_read = on_client_read;
+    server->on_close = on_client_close;
+    while (true) {
+        if (net_socket_select(server)) {
+            printf("Handled all events.\n");
+        } else {
+            printf("No events to handle.\n");
+        }
+    }
+    return 0;
+}