fix: replace direct stdio calls with portable I/O hooks

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

forth_core.c

@@ -78,7 +78,7 @@ void data_push(int64_t val) {
 
 int64_t data_pop(void) {
     if (data_sp < 0) {
-        fprintf(stderr, "Data stack underflow\n");
+        forth_printf("Data stack underflow\n");
         return 0;
     }
     return data_stack[data_sp--];

forth_interp.c

@@ -87,15 +87,21 @@ void outer_interpreter(void) {
     while (1) {
         forth_printf("ok ");
         forth_fflush();
-        if (fgets(line_buf, sizeof(line_buf), stdin) == NULL) {
+        size_t i = 0;
+        int c;
+        while (i < sizeof(line_buf) - 1) {
+            c = forth_getchar();
+            if (c == EOF || c == '\n' || c == '\r') {
+                break;
+            }
+            line_buf[i++] = (char)c;
+        }
+        line_buf[i] = '\0';
+        if (i == 0 && c == EOF) {
             break;
         }
         input_buf = line_buf;
         input_buf_cap = sizeof(line_buf);
-        size_t n = strlen(input_buf);
-        if (n > 0 && input_buf[n - 1] == '\n') {
-            input_buf[n - 1] = '\0';
-        }
         input_ptr = input_buf;
         char* tok;
         while ((tok = next_token()) != NULL) {