fix: fallback key in multi-key mode + bound printf context

- decrypt_images.c: try image_keys.json lookup first, fall back to
  config.json single key when CT pattern not mapped (previously returned
  -5 immediately in multi-key mode)
- find_image_key.c: cap ASCII context printf to remaining buffer length,
  preventing out-of-bounds read near region end

find_image_key.c

@@ -470,8 +470,10 @@ static int scan_pid(pid_t pid) {
                                         printf("\n  *** FOUND KEY: %s ***\n", kh);
                                         printf("      Pattern: %s (%d files)\n",
                                                ch, patterns[idx].file_count);
-                                        printf("      ASCII context: %.32s\n",
-                                               buf + run_start);
+                                        int ctx_len = data_cnt - run_start;
+                                        if (ctx_len > 32) ctx_len = 32;
+                                        printf("      ASCII context: %.*s\n",
+                                               ctx_len, buf + run_start);
                                         found_this_pid++;
                                         /* Rebuild */
                                         n_unsolved = 0;