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.rhashtable.c

@@ -0,0 +1,12 @@
+#include "rhashtable.h"
+#include "rtest.h"
+#include "rstring.h"
+
+int main() {
+
+    for (int i = 0; i < 1000; i++) {
+        char *key = rgenerate_key();
+        rset(key, "tast");
+        rasserts(!strcmp(rget(key), "tast"));
+    }
+}