feat(d3d11): implement SwapDeviceContextState

[WIndFate]

Summary

SwapDeviceContextState was UNIMPLEMENTED. Callers such as Direct2D interop use it to swap their own pipeline state in and the application's state out around each render pass, so any such render loop aborts at the first call (e.g. Wine's Direct2D at the first Clear).

DXMT already has the pieces needed: a single D3D11ContextState struct, the context's state_, ResetEncodingContextState() and InvalidateRenderPipeline(). This change:

• adds a D3D11ContextState saved_state slot to MTLD3D11DeviceContextState
• saves the current state_ into the returned previous-state token
• activates the incoming token's saved state
• resets encoding state and invalidates the render pipeline so the Metal side follows the new state

Testing

Verified on macOS with a CrossOver/Wine runtime on an Apple M-series GPU. Together with the IDXGISurface change (#171), a Wine Direct2D render loop (BeginDraw → Clear → draw → EndDraw) runs ~150 frames with EndDraw returning S_OK and no errors in the log.

Notes

Builds on the existing context-state machinery rather than introducing new state tracking.