Refactor live shadow settings transaction

Clean up live shadow application without changing renderer behavior. Separate engine validation/lifecycle/commit concerns from renderer preparation and commit. Keep device-loss, out-of-memory, rollback-by-construction, shader replacement, and resource-retirement semantics intact. Validate with focused graphics and shadow tests when a .NET SDK is available. Validation blocker: this environment has no `dotnet` executable or installed SDK, so the focused test suite cannot be run here.