Move particles between ticks

The particle library moves particles by their velocity times a tick's time at
each tick; between ticks, the adapter (ModelInstance::render's call to
ParticleSystem::render, redirected after checking it) draws them moved on by
their velocity times the time since, then moves each back by its own velocity:
ParticleSystem::render sorts them by distance, reordering them in memory, so
restoring saved positions in order scrambled them. Like ParticleSystem::render,
it only reads particles when there's a group (the library throws without one,
which crashed the client).

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

docs/architecture.md

@@ -97,7 +97,9 @@ their animations a fraction of a frame on (ModelInstance::render's call to updat
 between keyframes; animations held, looping or starting are posed as they are), and the camera where its easing (a
 share 0.4 of the way to its target per step) has brought it, `1 - 0.6^fraction` of the way (CameraController's
 getCameraTransform, in its exported vtable). Nothing is predicted: both are what the next step would give, at the
-fraction. Still at 30 per second: particles, crossfades between animations, gaze.
+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.
 
 Next: sharing pipelines between views (each view's scene compiles its own).