Add Microsoft.Extensions.DependencyInjection package and introduce an
IStartupTracer abstraction with NoOpStartupTracer (zero-overhead default)
and FileStartupTracer (renamed from WinUiStartupTrace). A StartupTracer
factory selects the implementation from environment configuration.
Consumers (App, MainWindow, MainWindowViewModel, SensorColumnMeasurer) now
depend on the non-nullable interface, removing null-conditional trace calls
and decoupling startup instrumentation from construction logic. No behavior
change: tracing still writes the same log when enabled.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
JMR-devandCopilot