fix(desktop): persist the save when a ROM window ends on a diagnostic

A core, DMA, rendering, window, or audio error returned before the save was
written, so every completed in-game save since launch was lost. The window now
writes changed bytes on that path too. The diagnostic stays the session error;
a refused or failed write is reported as output. A failed state restore leaves
save memory undefined and is never written. Terminal runs keep the strict rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU

crates/desktop/src/main.rs

@@ -141,7 +141,7 @@ fn run() -> Result<(), Box<dyn Error>> {
                 --audio                  Enable macOS output at 1x (default: muted)\n\
                 --rtc                    Select cartridge RTC (UTC startup, host elapsed time)\n\
                 --save-type flash64|flash128  Select nominal Macronix Flash emulation\n\
-                --save-file PATH         Load raw Flash image; persist only on clean exit\n\
+                --save-file PATH         Load raw Flash image; persist on clean exit or window diagnostic\n\
                 --state-file PATH        ROM window save state; F5 saves, F9 loads (default: ROM path + .state)\n\
                 --test-suite PATH.json    Run bounded ROM assertions; write a JSON report\n\
                 --cpu-demo     Run the terminal-only ARM/Thumb instruction demo\n\