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/NodeRegistry.ts

@@ -19,10 +19,11 @@ export interface PortDef {
 }
 
 /**
- * `state` and `shader` are the categories no behavior graph offers: their nodes are the states of
- * a state machine and the GLSL of a shader asset.
+ * `state`, `shader` and `particles` are the categories no behavior graph offers: their nodes are the
+ * states of a state machine, the GLSL of a shader asset and the motion of one particle.
  */
-export type NodeCategory = 'event' | 'flow' | 'data' | 'variable' | 'action' | 'state' | 'shader';
+export type NodeCategory =
+  'event' | 'flow' | 'data' | 'variable' | 'action' | 'state' | 'shader' | 'particles';
 
 /**
  * How a loop node asks for one of its exec outputs to be run repeatedly: `port` carries the body,