fix: qdup/tuck/2swap bugs, getline, and cached word pointers

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

forth_core.c

@@ -18,7 +18,8 @@ int32_t compile_idx = 0;
 int32_t compile_cap = 0;
 char compiling_name[MAX_NAME_LEN + 1] = {0};
 
-char input_buf[1024] = {0};
+char* input_buf = NULL;
+size_t input_buf_cap = 0;
 char* input_ptr = NULL;
 
 int64_t *compile_stack = NULL;
@@ -29,6 +30,13 @@ Cell *user_mem = NULL;
 int64_t user_mem_size = 0;
 Cell* here = NULL;
 
+Word* w_exit = NULL;
+Word* w_docolon = NULL;
+Word* w_lit = NULL;
+Word* w_branch = NULL;
+Word* w_zbranch = NULL;
+Word* w_dot_quote_inner = NULL;
+
 // ---------- Data stack ----------
 void data_push(int64_t val) {
     if (data_sp + 1 >= data_cap) {