feat: abstract I/O and strings for freestanding compilation

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

platform_hosted.c

@@ -0,0 +1,38 @@
+#include "forth.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+#include <stdarg.h>
+
+void forth_putchar(char c) {
+    putchar((unsigned char)c);
+}
+
+int forth_getchar(void) {
+    return getchar();
+}
+
+void forth_fflush(void) {
+    fflush(stdout);
+}
+
+void forth_panic(void) {
+    for (;;);
+}
+
+int64_t forth_strtoll(const char* str, char** endptr, int base) {
+    return (int64_t)strtoll(str, endptr, base);
+}
+
+void forth_printf(const char* fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    vprintf(fmt, ap);
+    va_end(ap);
+}
+
+int main(void) {
+    forth_run();
+    return 0;
+}