How do I use a seeded RNG so a spin is exactly reproducible?

Inject a deterministic generator with .rng(fn) on the builder. The engine takes any () => number in [0, 1), so bring your own PRNG -- the repo's SeededRng is example code (@pixi-reels/cheats/seededRng, a private workspace package), not part of the published package, so copy it in rather than importing it from pixi-reels.