test: differential st_blocks for --sparse/--preallocate and --ignore-existing wire volume

- Assert FastSync's sparse/preallocate block accounting matches rsync 3.4.1
  for a hole file (preallocate wins over sparse, exactly like rsync).
- Assert --ignore-existing is answered by the receiver before the sender
  transmits the payload (CountingProxy wire volume near-zero).
- CountingProxy.run gains a bounded join_timeout so tests that only need the
  client->server count do not wait for the server's idle socket.
- Fix the stale protocol 2.24.0 comment in test_config.c (golden is 2.26.0).

tests/test_config.c

@@ -2920,7 +2920,7 @@ static unsigned long long capture_wire_hash(const Config* cfg, size_t* out_len)
   return h;
 }
 
-/* Byte-for-byte wire compatibility guard (protocol 2.24.0).  The expected hash
+/* Byte-for-byte wire compatibility guard (protocol 2.26.0).  The expected hash
  * pins the pre-X-macro byte stream; the refactor MUST NOT change it. */
 static void test_config_wire_golden() {
   if (is_running_under_valgrind())