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.h

@@ -0,0 +1,67 @@
+#ifndef RHASHTABLE_H
+#define RHASHTABLE_H
+/*
+    ORIGINAL SOURCE IS FROM K&R
+ */
+
+#include "rmalloc.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define HASHSIZE 101
+
+// Structure for the table entries
+typedef struct rnlist {
+    struct rnlist *next;
+    char *name;
+    char *defn;
+} rnlist;
+
+// Hash table array
+static rnlist *rhashtab[HASHSIZE];
+
+// Hash function
+unsigned rhash(char *s) {
+    unsigned hashval;
+    for (hashval = 0; *s != '\0'; s++)
+        hashval = *s + 31 * hashval;
+    return hashval % HASHSIZE;
+}
+
+rnlist *rlget(char *s) {
+    rnlist *np;
+    for (np = rhashtab[rhash(s)]; np != NULL; np = np->next)
+        if (strcmp(s, np->name) == 0)
+            return np; // Found
+    return NULL;       // Not found
+}
+
+// Lookup function
+char *rget(char *s) {
+    rnlist *np = rlget(s);
+    return np ? np->defn : NULL;
+}
+
+// Install function (adds a name and definition to the table)
+struct rnlist *rset(char *name, char *defn) {
+    struct rnlist *np = NULL;
+    unsigned hashval;
+
+    if ((rlget(name)) == NULL) { // Not found
+        np = (struct rnlist *)malloc(sizeof(*np));
+        if (np == NULL || (np->name = strdup(name)) == NULL)
+            return NULL;
+        hashval = rhash(name);
+        np->next = rhashtab[hashval];
+        rhashtab[hashval] = np;
+    } else {
+        if (np->defn)
+            free((void *)np->defn);
+        np->defn = NULL;
+    }
+    if ((np->defn = strdup(defn)) == NULL)
+        return NULL;
+    return np;
+}
+#endif