feat: abstract I/O and strings for freestanding compilation

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

forth.h

@@ -1,29 +1,46 @@
 #ifndef FORTH_H
 #define FORTH_H
 
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <ctype.h>
 #include <stdint.h>
-#include <inttypes.h>
+#include <stddef.h>
+#include <stdarg.h>
 
 // Configuration (all hard limits removed)
 #define MAX_NAME_LEN 31
+#define FORTH_EOF (-1)
 
-/* Portable panic hook. Override for bare-metal reset/handling. */
-#ifndef forth_panic
-#define forth_panic() do { for (;;) ; } while (0)
-#endif
+/* Platform interface -- must be provided by the target platform layer. */
+void forth_putchar(char c);
+int forth_getchar(void);
+void forth_printf(const char* fmt, ...);
+void forth_fflush(void);
+void forth_panic(void);
 
-/* Portable I/O hooks. Define FORTH_CUSTOM_IO before including this header
- * to provide bare-metal UART (or other) implementations. */
-#ifndef FORTH_CUSTOM_IO
-#define forth_putchar(c) putchar(c)
-#define forth_getchar() getchar()
-#define forth_printf(...) printf(__VA_ARGS__)
-#define forth_fflush() fflush(stdout)
-#endif
+/* Platform string-to-number (base 10 is required). */
+int64_t forth_strtoll(const char* str, char** endptr, int base);
+
+/* Portable character classification. */
+static inline int forth_isspace(int c) {
+    return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f' || c == '\v';
+}
+
+/* Portable string operations. */
+static inline size_t forth_strlen(const char* s) {
+    size_t n = 0;
+    while (s[n]) n++;
+    return n;
+}
+
+static inline void forth_memcpy(void* dst, const void* src, size_t n) {
+    char* d = dst;
+    const char* s = src;
+    while (n--) *d++ = *s++;
+}
+
+static inline int forth_strcmp(const char* a, const char* b) {
+    while (*a && *a == *b) { a++; b++; }
+    return (unsigned char)*a - (unsigned char)*b;
+}
 
 /* Static memory sizes (override before including forth.h) */
 #ifndef FORTH_DATA_STACK_SIZE
@@ -105,6 +122,9 @@ extern Word* w_branch;
 extern Word* w_zbranch;
 extern Word* w_dot_quote_inner;
 
+// Entry point
+void forth_run(void);
+
 // Core function prototypes
 void data_push(int64_t val);
 int64_t data_pop(void);