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>

README.md

@@ -37,7 +37,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 816 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 821 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs