fix: replace unsafe fixed-size buffers with dynamic formatting helpers; add util format helpers; centralize log_on_line cleanup

v0/util.c

@@ -0,0 +1,27 @@
+#include "util.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+char* format_string_va(const char* fmt, va_list args) {
+    if (!fmt) return NULL;
+    va_list args_copy;
+    va_copy(args_copy, args);
+    int needed = vsnprintf(NULL, 0, fmt, args_copy);
+    va_end(args_copy);
+    if (needed < 0) return NULL;
+
+    char* buf = (char*)malloc((size_t)needed + 1);
+    if (!buf) return NULL;
+    vsnprintf(buf, (size_t)needed + 1, fmt, args);
+    return buf;
+}
+
+char* format_string(const char* fmt, ...) {
+    if (!fmt) return NULL;
+    va_list args;
+    va_start(args, fmt);
+    char* s = format_string_va(fmt, args);
+    va_end(args);
+    return s;
+}