CVE-2026-29518: fix daemon-no-chroot TOCTOU on parent path components by
routing both the sender read-path (sender.c) and the receiver write-path
(receiver.c open_tmpfile and recv_files inplace branch) through
secure_relative_open() / secure_mkstemp() from the trusted module root,
and add regression test (chdir-symlink-race.test case 4)