Refactor live display settings transaction

Clean up `Engine.TryApplyDisplaySettings` without changing live display behavior. Acceptance criteria: - Display validation, lifecycle gating, native-window application, presentation refresh, commit, and rollback are expressed as separate responsibilities. - Recoverable presentation failures restore the prior window and presentation state; unsafe rollback failures still propagate. - Display settings code is separated from unrelated `Engine.cs` concerns where practical. - Existing display-option tests pass, with focused regression coverage for extracted logic where feasible. Validation blocker: this environment has no `dotnet` executable or installed SDK, so the focused test suite cannot be run here.