feat: abstract I/O and strings for freestanding compilation

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

main.c

@@ -1,6 +1,6 @@
 #include "forth.h"
 
-int main(void) {
+void forth_run(void) {
     here = user_mem;
 
     // Hidden words first
@@ -104,5 +104,4 @@ int main(void) {
 
     // Start outer interpreter
     outer_interpreter();
-    return 0;
 }