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/src/main.rs

@@ -12,6 +12,7 @@
 #![forbid(unsafe_code)]
 
 mod control_server;
+mod terminate;
 mod update;
 mod usage;
 
@@ -267,6 +268,14 @@ fn main() -> eframe::Result {
                 log::info!("graphics {:?} {} ({}), MSAA {}x (requested {MSAA}x)", info.backend, info.name, info.driver, rs.msaa_samples);
                 app.set_wgpu(rs, DEPTH_BITS, rs.msaa_samples, &cc.egui_ctx);
             }
+            // `kill`, Ctrl-C or a logout closes the window as Close does: autosaved, no crash (#248).
+            let ctx = cc.egui_ctx.clone();
+            if let Err(e) = terminate::watch(move || {
+                ctx.send_viewport_cmd(egui::ViewportCommand::Close);
+                ctx.request_repaint();
+            }) {
+                log::warn!("termination signals: {e}");
+            }
             if let Some(port) = args.control {
                 app = app.with_control(control_server::start(port, cc.egui_ctx.clone()));
             }