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/ui/Theme.ts

@@ -56,6 +56,7 @@ export const Theme = {
     categoryAction: 0x2f6fd6,
     categoryState: 0x8e5ad6,
     categoryShader: 0x1f8f8f,
+    categoryParticles: 0xc26a1f,
     portExec: 0xffffff,
     portNumber: 0x6fcf97,
     portBoolean: 0xe57373,