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.rrex3.c

@@ -0,0 +1,10 @@
+
+#include "rrex3.h"
+#include "rtest.h"
+
+int main() {
+    printf("Testing rrex3 regular expression parser.");
+    rrex3_test();
+    return 0;
+    // return rtest_end("");
+}