merge: resolve dev (quality refactor) into security-fixes

- file.c split layout retained; security-hardened secure-fs helpers
  (open_secure_parent/to_disk_secure/rename_secure/stat_secure) now live in
  file.c with file_ prefix and are shared with file_receive.c
- file_receive.c takes the security branch's bounded allocations
  (receive_data_limited, data_decompress_limited, size checks) and
  STATUS_ERROR signaling
- file_send.c gains the data consistency check on file->data
- client_validation.c: stricter --tls requiring --ca, log_message style
- utils.c: hardened openat/mkdirat mkdir_r from security branch

tests/test_property.c

@@ -88,7 +88,7 @@ static void test_property_chunk_roundtrip() {
     for (int i = 0; i < content_len; i++)
       content[i] = (char)(rand() % 256);
 
-    to_disk(path, content, content_len, false, false);
+    file_write_to_disk(path, content, content_len, false, false);
 
     struct stat st;
     stat(path, &st);