feat: abstract I/O and strings for freestanding compilation

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

forth_interp.c

@@ -3,7 +3,7 @@
 // Input tokenizer (unchanged)
 char* next_token(void) {
     if (input_ptr == NULL) return NULL;
-    while (*input_ptr != '\0' && isspace((unsigned char)*input_ptr)) {
+    while (*input_ptr != '\0' && forth_isspace((unsigned char)*input_ptr)) {
         input_ptr++;
     }
     if (*input_ptr == '\0') return NULL;
@@ -63,10 +63,10 @@ void process_token(const char* token) {
         }
     } else { // Try to parse as number
         char* end;
-        long long v = strtoll(token, &end, 10);
+        int64_t v = forth_strtoll(token, &end, 10);
         if (end != token && *end == '\0') {
             if (state == 0) {
-                data_push((int64_t)v);
+                data_push(v);
             } else { // Compile lit + number
                 if (!w_lit) {
                     forth_printf("Fatal: lit word not found\n");
@@ -91,13 +91,13 @@ void outer_interpreter(void) {
         int c;
         while (i < sizeof(line_buf) - 1) {
             c = forth_getchar();
-            if (c == EOF || c == '\n' || c == '\r') {
+            if (c == FORTH_EOF || c == '\n' || c == '\r') {
                 break;
             }
             line_buf[i++] = (char)c;
         }
         line_buf[i] = '\0';
-        if (i == 0 && c == EOF) {
+        if (i == 0 && c == FORTH_EOF) {
             break;
         }
         input_buf = line_buf;