linux: terminate on SIGTERM/SIGINT in the host scaffold and example

[turinglambdaai]

Problem

Reproduced on Fulcrum 0.4.1 (Ubuntu 26.04, GNOME/Wayland): kill -TERM on a running app broke the embedded Racket backend (Rivet backend terminated: user break) while the GTK process stayed alive. The window remained, every search silently did nothing, and only kill -9 removed it. systemd --user stop, session logout, and killall all send exactly this signal.

Root cause

The Linux system adapter has shipped InstallShutdownHook since #128 with a solid self-pipe + watcher design (covered by --shutdown-probe), but nothing outside the integration probe installs it. The GTK shutdown signal never fires on an OS signal, so the orderly cleanup path is unreachable for terminations.

Change

• platform/linux/host/src/main.cpp (the scaffold every rivet-new app copies): refactor the GTK shutdown cleanup into perform_orderly_shutdown and install it via InstallShutdownHook in main.
• examples/taskboard/linux/src/main.cpp: same wiring.
• Hook installation failure degrades to a stderr note — apps still run, just without signal handling.

Verification

• Template host builds clean against a local Racket 9.3 CS + GTK4 toolchain.
• RivetIntegration --shutdown-probe: TERM → callback runs (marker written) → exit code 0.

Apps with their own copies of the scaffold (Fulcrum, SyncPilot) need the same one-liner in their linux/src/main.cpp — follow-ups will land in those repos.