Since 6d4e851d8 the serve startup path imports tui_gateway.server (for the
flush-on-SIGTERM handlers) before the READY sentinel is printed. That module
redirects sys.stdout to sys.stderr at import time, so the
HERMES_(BACKEND|DASHBOARD)_READY port=<n> sentinel landed on stderr while the
Electron desktop spawn watches child.stdout only — the desktop timed out
after 90s and killed a perfectly healthy backend (issue #96282).
Write the sentinel to the real stdout file descriptor (fd 1 is untouched by
the Python-level redirect), with a print() fallback.
Adds a regression test that captures stdout/stderr separately — the existing
E2E suite merges them, which is exactly how this slipped past CI.
f2dd32d3e5