feat: add comma, fix allot, and support bare-metal custom memory

Co-authored-by: aider (openrouter/moonshotai/kimi-k2.6) <aider@aider.chat>

forth_words.c

@@ -456,13 +456,25 @@ void do_here(Word* w) {
 void do_allot(Word* w) {
     (void)w;
     int64_t n = data_pop();
-    if (here + n > user_mem + user_mem_size) {
-        forth_printf("User memory overflow\n");
+    int64_t idx = here - user_mem;
+    if (idx + n < 0 || idx + n > user_mem_size) {
+        forth_printf("User memory out of bounds\n");
         return;
     }
     here += n;
 }
 
+void do_comma(Word* w) {
+    (void)w;
+    int64_t val = data_pop();
+    if (here + 1 > user_mem + user_mem_size) {
+        forth_printf("User memory overflow\n");
+        return;
+    }
+    here->num = val;
+    here++;
+}
+
 // Variable and constant
 void do_variable(Word* w) {
     (void)w;