perf: lightweight performance fixes (CA1859, CA1869, CA1308, CA2012, CA1819, CA5392)

[DeepDiver1975]

• CA1859 (3): IncidentStore._fileStore typed as IncidentFileStore; AtemschutzTrupp.ValidateCrew takes List (Register already passes members.ToList()); MasterDataStore.SettingsRows returns (string Key, int Value)[] instead of IEnumerable.
• CA1869 (1): MasterDataJson caches a single static JsonSerializerOptions instead of allocating one per Serialize call.
• CA1308 (1): FilesViewModel.ContentTypeFor normalizes extension with ToUpperInvariant (arms uppercased).
• CA2012 (1): MainWindowViewModel awaits ws.LeaveAsync() in the GoHome action instead of discarding the ValueTask (async void lambda, matching the Action-typed event semantics).
• CA5392 (1): SystemAlarmService.PlaySound gets DefaultDllImportSearchPaths(System32).
• CA1819 (1): AddFileCommand.Bytes suppressed with justification — byte[] is the wire form System.Text.Json base64-encodes.
• Verification: AnalysisMode=All harness shows all six rules at 0, zero errors, no other rule delta vs main; full suite 801 passed / 1 skipped (unchanged).