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/viewport/ParticleLibrary.ts

@@ -0,0 +1,40 @@
+import type { ParticlesAsset } from '../core/model/Asset';
+import { compileParticles, type ParticlesBuild, particlesFingerprint } from '../core/particles/compile';
+
+/**
+ * Compiled particle graphs, one per asset, recompiled when the graph's content changes – the same
+ * contract the `ShaderLibrary` keeps, with a clock of its own so a test can hold particle time
+ * still without freezing the shaders too.
+ */
+export class ParticleLibrary {
+  private readonly builds = new Map<string, { fingerprint: string; build: ParticlesBuild }>();
+  /** How often a graph went through the compiler – what the checks read to prove a move did not. */
+  compiles = 0;
+  private readonly started = performance.now();
+  private frozenAt: number | null = null;
+
+  /** Seconds since the library came up – the clock every source's age is measured on. */
+  now(): number {
+    return this.frozenAt ?? (performance.now() - this.started) / 1000;
+  }
+
+  freezeTime(at: number | null): void {
+    this.frozenAt = at;
+  }
+
+  get frozen(): number | null {
+    return this.frozenAt;
+  }
+
+  buildFor(asset: ParticlesAsset): ParticlesBuild {
+    const fingerprint = particlesFingerprint(asset.graph);
+    const cached = this.builds.get(asset.id);
+    if (cached && cached.fingerprint === fingerprint) return cached.build;
+    this.compiles += 1;
+    const build = compileParticles(asset.graph);
+    this.builds.set(asset.id, { fingerprint, build });
+    return build;
+  }
+}
+
+export const particleLibrary = new ParticleLibrary();