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]>

docs/architecture.md

@@ -99,7 +99,8 @@ share 0.4 of the way to its target per step) has brought it, `1 - 0.6^fraction`
 getCameraTransform, in its exported vtable). Nothing is predicted: both are what the next step would give, at the
 fraction; and particles moved on by their velocity times the time since the step (ModelInstance::render's call to
 ParticleSystem::render, redirected), then back by the same (the client sorts them by distance as it draws them, which
-reorders them). Still at 30 per second: crossfades between animations, gaze.
+reorders them); and crossfades between animations (weights ramped in tenths over 10 steps by ModelInstance::step),
+partway to the next step's weights. Still at 30 per second: gaze (heads turning to whoever's talking).
 
 Next: sharing pipelines between views (each view's scene compiles its own).