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.rtypes.c

@@ -0,0 +1,14 @@
+#include "rtest.h"
+#include "rtypes.h"
+
+int main() {
+    rtest_banner("rtypes");
+    uint test1 = 1000;
+    rassert(test1 == 1000) ulong test2 = -1000;
+    rassert(test2 == -1000);
+    byte test3 = 123;
+    rassert(test3 == 123);
+    byte test4 = 'A';
+    rassert(test4 == 65);
+    return rtest_end("");
+}