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.

src/main/desktop-diagnostics.ts

@@ -13,6 +13,10 @@ const DEFAULT_MAXIMUM_FILES = 4
 const DEFAULT_MAXIMUM_RECORDS = 1_000
 export const MAXIMUM_PENDING_DESKTOP_DIAGNOSTIC_WRITES = 32
 const DIAGNOSTIC_FILE_NAME = 'desktop-diagnostics.ndjson'
+const rendererExitReasons = new Set([
+  'clean-exit', 'abnormal-exit', 'killed', 'crashed', 'oom',
+  'launch-failed', 'integrity-failure'
+])
 const allowedStages = new Set([
   'renderer',
   'startup',
@@ -69,6 +73,8 @@ export type DesktopDiagnosticFailure = Readonly<{
   stage: string
   code: string
   error: unknown
+  reason?: string
+  exitCode?: number
 }>
 
 export type DesktopDiagnosticFailureObserver = (
@@ -82,6 +88,8 @@ export type DesktopDiagnosticRecord = Readonly<{
   code: string
   errorType: string
   message: string
+  reason?: string
+  exitCode?: number
 }>
 
 export type DesktopDiagnosticsOptions = Readonly<{
@@ -169,13 +177,20 @@ export function normalizeDesktopDiagnosticRecord(
   }
   const component = safeComponent(candidate.component)
   const stage = safeStage(candidate.stage)
+  const code = safeCode(candidate.code)
+  const rendererGone = component === 'desktop' && stage === 'renderer' &&
+    code === 'desktop.renderer.gone'
   return Object.freeze({
     timestamp: timestamp.toISOString(),
     component,
     stage,
-    code: safeCode(candidate.code),
+    code,
     errorType: safeStoredErrorType(candidate.errorType),
-    message: fixedMessage(component, stage)
+    message: fixedMessage(component, stage),
+    ...(rendererGone && typeof candidate.reason === 'string' &&
+      rendererExitReasons.has(candidate.reason) ? { reason: candidate.reason } : {}),
+    ...(rendererGone && Number.isSafeInteger(candidate.exitCode)
+      ? { exitCode: candidate.exitCode } : {})
   })
 }
 
@@ -249,7 +264,9 @@ export class DesktopDiagnostics {
         component: failure.component,
         stage: failure.stage,
         code: failure.code,
-        errorType: safeErrorType(failure.error)
+        errorType: safeErrorType(failure.error),
+        reason: failure.reason,
+        exitCode: failure.exitCode
       })
       if (normalized === undefined) {
         return Promise.reject(