A particle is a function of its age, not a thing that is stepped (#73)

The asset type `particles` is placeable and carries a graph at the same time —
`behavior` was the blueprint. Because a continuous emitter gives birth to particle n
at time n/rate, the whole field is a function of the clock rather than of the frames:
no ring-buffer bookkeeping, exact time jumps, reproducibility as a measurable property.

compile.ts follows compileShader's frame but emits JS closures into a Float64Array
instead of GLSL, and without eval; a node with two consumers is evaluated once.
Drawing is one Pixi ParticleContainer per source, so one draw call.

Collision, wind and braking in flight are out for good — that is the price of the
formula model, not an omission.

The tab count goes from nine to ten, which is exactly the case #84 was built for one
day earlier: at 1280 px the row no longer holds every tab, so guard 14b now asks for
hidden.length > 0 instead of === 0.

src/core/graph/NodeContext.ts

@@ -26,6 +26,11 @@ export interface EntityHandle {
   z: number;
   props: PropertyBag;
   parent: EntityHandle | null;
+  /**
+   * Counts the Emit pulses a particle source has been given. Kept off `props` for the reason the
+   * animation clock is: what the world writes back to the model must not grow an override per frame.
+   */
+  emits?: number;
 }
 
 /** Position, rotation, scale and mirror of every ancestor applied, nearest last. */