feat: add depth, pick, roll, ?dup, 2dup, 2drop, and 2swap words

Co-authored-by: aider (openrouter/deepseek/deepseek-v4-pro) <aider@aider.chat>

forth.h

@@ -166,4 +166,13 @@ void do_until(Word* w);
 void do_while(Word* w);
 void do_repeat(Word* w);
 
+/* Additional stack words */
+void do_depth(Word* w);
+void do_pick(Word* w);
+void do_roll(Word* w);
+void do_qdup(Word* w);
+void do_2dup(Word* w);
+void do_2drop(Word* w);
+void do_2swap(Word* w);
+
 #endif

main.c

@@ -99,6 +99,15 @@ int main(void) {
     add_primitive("while", do_while, 1 << 7);
     add_primitive("repeat", do_repeat, 1 << 7);
 
+    /* Additional words */
+    add_primitive("depth", do_depth, 0);
+    add_primitive("pick",  do_pick, 0);
+    add_primitive("roll",  do_roll, 0);
+    add_primitive("?dup",  do_qdup, 0);
+    add_primitive("2dup",  do_2dup, 0);
+    add_primitive("2drop", do_2drop, 0);
+    add_primitive("2swap", do_2swap, 0);
+
     // Start outer interpreter
     outer_interpreter();
     return 0;