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_compression.c

@@ -64,8 +64,8 @@ static void test_chunk_compress_decompress_roundtrip() {
   char* content2 = "chunk compression test file 2 with more data";
   unsigned long long len2 = strlen(content2);
 
-  to_disk(path1, content1, len1, false, false);
-  to_disk(path2, content2, len2, false, false);
+  file_write_to_disk(path1, content1, len1, false, false);
+  file_write_to_disk(path2, content2, len2, false, false);
 
   struct stat st1, st2;
   EXPECT_EQ_INT(stat(path1, &st1), 0);