Time is an argument, so speed scales it and a prewarm offsets it (#87)

Speed and Prewarm are not state and not a second loop: both are arithmetic
on the clock argument the field already passes to every particle formula.
fieldTime = elapsed * timeScale + prewarmOffset, a pure rule next to
liveRange, called at the one place ParticleField.sync forms its time, so
preview and world inherit it together.

Prewarm offsets by one lifetime because that is the smallest offset which
fills a continuous emitter; a burst is dead at time >= lifetime, so the
same offset would skip it entirely and it is suppressed there.

The tab's slider counts wall-clock seconds, so its span divides by the
scale. A missing row falls back to 1 / off, so old project files need no
migration.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/particles/ParticlesEditor.ts

@@ -1,5 +1,5 @@
 import type { Asset, ParticlesAsset } from '../../core/model/Asset';
-import { LIFETIME_KEY } from '../../core/properties/particles';
+import { LIFETIME_KEY, TIME_SCALE_KEY } from '../../core/properties/particles';
 import { defaultsFor } from '../../core/properties/PropertySchema';
 import { particleLibrary } from '../../viewport/ParticleLibrary';
 import { Button, clamp, HStack, Label, Slider, Spacer, Splitter, Theme, VStack, type Widget } from '../../ui';
@@ -151,10 +151,15 @@ export class ParticlesEditor extends AssetTabEditor<ParticlesAsset, GraphCanvas>
     this.syncSpan(asset);
   }
 
-  /** The slider covers three of the asset's own lifetimes, so a particle's whole arc fits. */
+  /**
+   * The slider covers three of the asset's own lifetimes, so a particle's whole arc fits – counted
+   * in the seconds the slider is in, which a time scale of 2 makes half as many.
+   */
   private syncSpan(asset: ParticlesAsset | null): void {
-    const lifetime = Number(asset?.props[LIFETIME_KEY] ?? defaultsFor('particles')[LIFETIME_KEY] ?? 1);
-    const span = Math.max(MIN_SPAN, lifetime * SPAN_LIFETIMES);
+    const props = { ...defaultsFor('particles'), ...asset?.props };
+    const lifetime = Number(props[LIFETIME_KEY] ?? 1);
+    const scale = Number(props[TIME_SCALE_KEY]);
+    const span = Math.max(MIN_SPAN, (lifetime * SPAN_LIFETIMES) / (scale > 0 ? scale : 1));
     if (span === this.span) return;
     this.span = span;
     this.slider.setRange(0, span);