test(listener): six processes share one selfSigned certificate from cold

ShouldShareSelfSignedCertificateQUIC: six processes, forked before any
thread or msquic exists and released together, each start two threads
that create a selfSigned listener at once against an empty cache
(XDG_CACHE_HOME pointed at a fresh directory) and with no openssl on
PATH, and connect to both. All twelve listeners serve, and all six
processes report the same certificate hash and a path under
XDG_CACHE_HOME. The directory is 0700 and the key 0600, nothing
temporary is left behind, a process started later serves the same
certificate, and a mismatched key planted on disk, what the race used
to leave, is replaced by the next process rather than loaded.

Against the previous implementation every child fails (`openssl:
command not found`), as do the location and permission checks. The
parent starts no thread, so forking stays safe, and its watchdog is
alarm(55); each child's is alarm(40).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

project.cpp

@@ -205,6 +205,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
             cfg.AddTest("ShouldSendRecieveQUICStream").Dependencies({ &cfg });
             cfg.AddTest("ShouldReportClosedQUICStream").Dependencies({ &cfg });
             cfg.AddTest("ShouldReportListenerPortQUIC").Dependencies({ &cfg });
+            cfg.AddTest("ShouldShareSelfSignedCertificateQUIC").Dependencies({ &cfg });
             cfg.AddTest("ShouldSurviveConnectionChurn").Dependencies({ &cfg });
         } else {
             // Only a --no-quic build can show that msquic stayed out.