fix: use bounded strlcpy/snprintf in arena.c

Finding triggers whenever there is a strcpy or strncpy used
Addresses c.lang.security.insecure-use-string-copy-fn.insecure-use-string-copy-fn

test/test_invariant_arena.c

@@ -0,0 +1,67 @@
+#include <check.h>
+#include <stdlib.h>
+#include <string.h>
+#include "../src/arena.c"
+
+START_TEST(test_buffer_reads_never_exceed_declared_length)
+{
+    // Invariant: Buffer reads never exceed the declared length
+    const char *payloads[] = {
+        "normal",                    // Valid input
+        "A",                         // Boundary: single char
+        "very_long_string_that_exceeds_buffer_by_more_than_double_the_size",  // Exploit case
+        "exact_length_plus_one",     // Boundary: length+1
+        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"  // Large payload
+    };
+    int num_payloads = sizeof(payloads) / sizeof(payloads[0]);
+    
+    for (int i = 0; i < num_payloads; i++) {
+        char dest[16] = {0};  // Fixed buffer size
+        const char *src = payloads[i];
+        size_t src_len = strlen(src);
+        
+        // Test strncpy behavior - should not write beyond dest[15]
+        strncpy(dest, src, sizeof(dest));
+        
+        // Verify no buffer overflow by checking last byte is either null or within bounds
+        ck_assert_msg(dest[sizeof(dest)-1] == '\0' || src_len < sizeof(dest),
+                     "Buffer overflow detected for payload: %s", src);
+        
+        // Additional safety check: ensure we can read entire buffer without segfault
+        for (size_t j = 0; j < sizeof(dest); j++) {
+            volatile char c = dest[j];  // Force read
+            (void)c;  // Suppress unused warning
+        }
+    }
+}
+END_TEST
+
+Suite *security_suite(void)
+{
+    Suite *s;
+    TCase *tc_core;
+
+    s = suite_create("Security");
+    tc_core = tcase_create("Core");
+
+    tcase_add_test(tc_core, test_buffer_reads_never_exceed_declared_length);
+    suite_add_tcase(s, tc_core);
+
+    return s;
+}
+
+int main(void)
+{
+    int number_failed;
+    Suite *s;
+    SRunner *sr;
+
+    s = security_suite();
+    sr = srunner_create(s);
+
+    srunner_run_all(sr, CK_NORMAL);
+    number_failed = srunner_ntests_failed(sr);
+    srunner_free(sr);
+
+    return (number_failed == 0) ? EXIT_SUCCESS : EXIT_FAILURE;
+}