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

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

forth_words.c

@@ -722,3 +722,70 @@ void do_repeat(Word* w) {
 
     compile_buf[while_offset_idx].num = compile_idx - while_offset_idx;
 }
+
+void do_depth(Word* w) {
+    (void)w;
+    data_push(data_sp + 1);
+}
+
+void do_pick(Word* w) {
+    (void)w;
+    if (data_sp < 0) return;
+    int64_t idx = data_pop();
+    if (idx < 0 || idx > data_sp) return;
+    int64_t value = data_stack[data_sp - (int32_t)idx];
+    data_push(value);
+}
+
+void do_roll(Word* w) {
+    (void)w;
+    if (data_sp < 0) return;
+    int64_t n = data_pop();
+    if (n == 0) return;
+    if (n < 0 || n > data_sp) return;
+    int64_t i = data_stack[data_sp - (int32_t)n];
+    int32_t pos = (int32_t)(data_sp - (int32_t)n);
+    while (pos < data_sp) {
+        data_stack[pos] = data_stack[pos + 1];
+        pos++;
+    }
+    data_stack[data_sp] = i;
+}
+
+void do_qdup(Word* w) {
+    (void)w;
+    if (data_sp < 0) return;
+    int64_t v = data_pop();
+    if (v != 0) {
+        data_push(v);
+        data_push(v);
+    }
+}
+
+void do_2dup(Word* w) {
+    (void)w;
+    if (data_sp < 1) return;
+    int64_t b = data_stack[data_sp];
+    int64_t a = data_stack[data_sp - 1];
+    data_push(a);
+    data_push(b);
+}
+
+void do_2drop(Word* w) {
+    (void)w;
+    data_pop();
+    data_pop();
+}
+
+void do_2swap(Word* w) {
+    (void)w;
+    if (data_sp < 3) return;
+    int64_t d = data_pop(); // x4
+    int64_t c = data_pop(); // x3
+    int64_t b = data_pop(); // x2
+    int64_t a = data_pop(); // x1
+    data_push(b);
+    data_push(a);
+    data_push(d);
+    data_push(c);
+}