fix: retain renderer exit details in diagnostic logs

Persist Electron renderer exit reasons and exit codes through diagnostic writes, reads, and exports so abnormal window exits can be investigated. Keep older records readable and existing log bounds unchanged.

Validation: 18 focused tests, typecheck, lint, and diff checks passed. Full test run timed out after 60 seconds with a SettingsPanel test failure.

Release note: Diagnostic logs now retain renderer exit reasons and exit codes to help distinguish out-of-memory exits from other window failures.

docs/development/window-recovery.md

@@ -21,8 +21,10 @@ diagnosis of their cause.
 - Native crashes and main-frame load failures use the existing bounded desktop
   diagnostics store (`desktop.renderer.gone`, `desktop.renderer.load-failed`,
   stage `renderer`). Persistent records retain fixed summaries rather than raw
-  errors. Crash reason/exit code and React errors/component stacks go to their
-  respective process consoles. There is no new diagnostic IPC.
+  errors. Renderer exit records also retain Electron's enumerated `reason` and
+  numeric `exitCode` when written, read back and exported; older records without
+  these fields remain readable. The exit details still go to the Main console.
+  React errors/component stacks remain console-only. There is no new diagnostic IPC.
 
 The boundary does not catch module evaluation/startup failures before React
 mounts, event-handler exceptions, asynchronous rejections, a hung renderer, or