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.rkeytable.h

@@ -0,0 +1,68 @@
+#ifndef RKEYTABLE_H
+#define RKEYTABLE_H
+/*
+    DERIVED FROM HASH TABLE K&R
+ */
+#include "rmalloc.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+
+typedef struct rnklist {
+    struct rnklist *next;
+    struct rnklist *last;
+    char *name;
+    char *defn;
+} rnklist;
+
+static rnklist *rkeytab = NULL;
+
+rnklist *rlkget(char *s) {
+    rnklist *np;
+    for (np = rkeytab; np != NULL; np = np->next)
+        if (strcmp(s, np->name) == 0)
+            return np; // Found
+    return NULL;       // Not found
+}
+
+char *rkget(char *s) {
+    rnklist *np = rlkget(s);
+    return np ? np->defn : NULL;
+}
+
+rnklist *rkset(char *name, char *defn) {
+    rnklist *np;
+    if ((np = (rlkget(name))) == NULL) { // Not found
+        np = (rnklist *)malloc(sizeof(rnklist));
+        np->name = strdup(name);
+        np->next = NULL;
+        np->last = NULL;
+
+        if (defn) {
+            np->defn = strdup(defn);
+        } else {
+            np->defn = NULL;
+        }
+
+        if (rkeytab == NULL) {
+            rkeytab = np;
+            rkeytab->last = np;
+        } else {
+            if (rkeytab->last)
+                rkeytab->last->next = np;
+
+            rkeytab->last = np;
+        }
+    } else {
+        if (np->defn)
+            free((void *)np->defn);
+        if (defn) {
+            np->defn = strdup(defn);
+        } else {
+            np->defn = NULL;
+        }
+    }
+    return np;
+}
+#endif