fix(client): route explicit remote dry-run targets, fail closed on stray status

--dry-run --server-host=H (or TLS / source-bind --address) silently ran the
client-side manifest even though a real run contacts the server.  Add a
client-only, never-serialized server_host_set bit (alongside the existing
server_port_set) and extend dry_run_targets_server so every explicit remote
target contacts the receiver.

Also make incremental_check return the dry-run code (4) only when the
session actually requested dry-run; a stray STATUS_DRY_RUN_TRANSFER from a
hostile/buggy peer is now a logged protocol error (STATUS_ERROR) instead of
falling through to send file data and desync.  Both normal send_single_file
callers handle rc == 4 explicitly as an abort.

src/shared/config.c

@@ -42,6 +42,7 @@ static void config_set_defaults(Config* config) {
   config->server_host = str_dup("127.0.0.1");
   config->server_port = 8080;
   config->server_port_set = false;
+  config->server_host_set = false;
   /* 0 means "--timeout not given": the transport keeps its own built-in 30 s
    * socket timeout (tcp_set_timeouts ignores non-positive values) and the
    * protocol layer keeps its built-in 60 s per-message deadline.  A positive