Crossfade animations between ticks

ModelInstance::step ramps the weights of animations fading in and out in tenths
over 10 ticks; between ticks, the posing hook sets them partway to the next
tick's weight, and puts them back with the times. Only if ModelInstance::step
counts the fades where expected (checked when installing); otherwise
crossfades stay at the client's rate.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

adapter/scene_hooks.hpp

@@ -8,7 +8,8 @@
 //   calls stepFrame, which steps each scene viewer). Recorded, for the frame pacer and for drawing between ticks.
 // - ModelInstance::render's call to updateCalSkeleton, which poses the model before it's drawn: between ticks, the
 //   model is posed at the time between them (its animations' times moved on by the fraction of a frame since the
-//   tick; Cal3D poses between keyframes), then its animations are put back as they were.
+//   tick; Cal3D poses between keyframes) and crossfading animations' weights partway to the next tick's, then its
+//   animations are put back as they were.
 // - ModelInstance::render's call to ParticleSystem::render: between ticks, particles drawn moved on by their velocity
 //   times the time since the tick (as the particle library moves them at each tick), then put back.
 // - CameraController::getCameraTransform (its exported vtable): between ticks, the camera where its easing toward its