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.

tests/core/graph.test.ts

@@ -74,8 +74,8 @@ describe('node registry', () => {
       expect(new Set(def.outputs.map((p) => p.key)).size).toBe(def.outputs.length);
       // An event is seeded by the runtime, a state is a name in a machine: neither runs.
       if (def.category === 'event' || def.category === 'state') expect(def.inputs).toHaveLength(0);
-      // A shader node is compiled to GLSL, never run.
-      else if (def.category === 'shader') expect('emit' in def).toBe(true);
+      // A shader node is compiled to GLSL and a particle node to a function: neither is ever run.
+      else if (def.category === 'shader' || def.category === 'particles') expect('emit' in def).toBe(true);
       else expect(def.run).toBeTypeOf('function');
       expect(keys.every((k) => k.length > 0)).toBe(true);
     }