app: a termination signal closes the window the normal way, not as a crash (#248)

SIGTERM, SIGINT and SIGHUP ended the process on the spot: no last autosave,
and the crash journal left behind became an unclean_exit report at the next
start. The report on #248's machine ("ended without a clean exit between
commands (last: control ui.capture)") has that exact signature and is
reproduced by kill -TERM after a capture; 500 captures of the solved
crank-mirror (50 app runs), 30 CLI snapshots and 250 MCP headless
screenshots exited cleanly. The first signal now asks the window to close;
a second ends the process with the signal's default action.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

apps/fieldcraft/Cargo.toml

@@ -25,6 +25,10 @@ ureq = { version = "3.4", default-features = false, features = ["rustls-no-provi
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
 getrandom = "0.3"
 
+# A termination signal (SIGTERM, SIGINT, SIGHUP) closes the window the normal way (terminate.rs).
+[target.'cfg(unix)'.dependencies]
+signal-hook = "0.3"
+
 [build-dependencies]
 # The Windows executable's icon resource (build.rs).
 embed-resource = "3.0.12"