refactor: make interpreter portable with static pools and I/O hooks

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

forth.h

@@ -7,7 +7,6 @@
 #include <ctype.h>
 #include <stdint.h>
 #include <inttypes.h>
-#include <sys/types.h>
 
 // Configuration (all hard limits removed)
 #define MAX_NAME_LEN 31
@@ -88,8 +87,11 @@ void inner_interpreter(void);
 void process_token(const char* token);
 void outer_interpreter(void);
 
-// Ensure compile_buf has at least 'needed' free cells
-void ensure_compile_cap(int32_t needed);
+// Ensure compile_buf has at least 'needed' free cells (returns 0 on overflow)
+int ensure_compile_cap(int32_t needed);
+
+Word* forth_alloc_word(void);
+Cell* forth_alloc_body(int32_t cells);
 
 // Stack ops
 void do_dup(Word* w);