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/editor/state.ts

@@ -376,7 +376,12 @@ export class EditorState {
     return this.selectedGraphId('shader');
   }
 
-  private selectedGraphId(type: 'behavior' | 'statemachine' | 'shader'): string | null {
+  /** The particle asset the selection refers to: the asset, a placed source, or one of its nodes. */
+  get selectedParticlesId(): string | null {
+    return this.selectedGraphId('particles');
+  }
+
+  private selectedGraphId(type: 'behavior' | 'statemachine' | 'shader' | 'particles'): string | null {
     const s = this.selection;
     if (!s) return null;
     if (s.kind === 'frame') return null;