v1.3.0

Added

• HeadlessProgram and HeadlessRunner for running the Elmish update loop without graphics, input polling, or Raylib initialization. Use for unit testing, server-side simulation, and CLI debugging.
• HeadlessProgram.mkHeadless init update — creates a headless program with the same Init/Update signatures as Program.
• HeadlessProgram builder DSL: withSubscribe, withTick, withFixedStep, withDispatchMode, withObserver.
• HeadlessProgram.observe — helper that creates a System.IObserver<'T> from an onNext callback, hiding the OnError/OnCompleted boilerplate.
• HeadlessRunner with explicit frame control: Step(TimeSpan), StepN(count, TimeSpan), StepUntil(predicate, TimeSpan, ?maxFrames).
• HeadlessRunner.Dispatch(msg) and DispatchMany(msgs) for sending messages from outside the update loop.
• HeadlessRunner.Model, GameTime, ShouldQuit for accessing simulation state.
• HeadlessRunner.Run(interval, ?ct) — returns seq<struct(GameTime * 'Model)>, a paced synchronous sequence of simulation frames. Uses spin-wait with Thread.Sleep(1) for timing.
• HeadlessRunner.RunAsync(interval, ct) — returns IAsyncEnumerable<struct(GameTime * 'Model)>, a paced async sequence of simulation frames. Uses PeriodicTimer for efficient timing.
• Observer support: HeadlessProgram.Observers field and withObserver DSL for registering System.IObserver<struct(GameContext * 'Model * GameTime)> factories. Observers fire every frame after the update loop, receiving the current model and game time. Observers implementing IDisposable are disposed when the runner is disposed.
• 27 unit tests for new features: step return values, observer lifecycle, observer correctness (post-update model, GameTime accumulation, multiple observers, window dimensions, subscription interaction), Run/RunAsync enumeration, cancellation, and ShouldQuit behavior. 47 total headless tests.
• XML documentation for HeadlessProgram.withTick, withFixedStep, and withDispatchMode.
• Headless mode documentation: Observers section (withObserver/observe), Run/RunAsync section with pacing and cancellation examples, server simulation example using observer-based broadcast.