v0.7.0: Let a node draw its own geometry and a seeded spray.

A node can draw its own geometry, and a spray keeps a seeded simulation
(ROADMAP task 15, stage 1). No shaders yet.

Added

• Custom::new(|ctx, t| { ... }). ctx.line, ctx.circle, ctx.rect and
  ctx.path take .fill or .stroke. ctx.rng(seed) does not advance
  random(), and the same seed draws the same shapes after a seek.
• Particles::new().seed(7).gravity(900.0).toward_camera() and
  burst(origin, count).await. Integration uses a fixed 1/120 s step and a
  checkpoint every 30 steps, so seeking back replays at most that many steps
  and lands on the same dots. toward_camera only scales the dots.
• oxframe check reports nondeterministic when painting the same frame
  twice does not match.
• Example spray: rain from Custom, then a burst into the camera.