Declarative camera shake: every impact in SceneShake::impacts is an
independent damped harmonic oscillator, summed together. Additive
over whatever super::scenario::Scene::camera already resolves to —
camera.keyframes is a single value-over-time series per property,
with no way to layer a second signal onto it without replacing the
first, so a shake needs its own field to coexist with a pan instead of
fighting over one keyframe track.
One beat-synced hit that kicks the camera. at resolves the same way
as any other in-scene TimePoint — relative to the scene’s own start
unless it carries the @ prefix (see the anchoring rule documented
once on TimePoint) — so an impact can land exactly on a cut:
{"at": "4b", "amplitude": 24.0}.