feat: add generic DiT support to spectrum cache

[rmatif]

Implementing the paper literally is heavier because it depends on architecture-specific internal feature hooks and final-block forecasting inside each DiT family, which would require invasive model-side changes and separate validation per model. This patch keeps Spectrum generic at the sampler level

Comparaison with easycache with 20 steps:

	Baseline	EasyCache	Spectrum
Image
Speedup	—	x1.82	x1.82
Config	—	0.2, 0.15, 0.95	default

[JohnLoveJoy]

I'd say it's better than baseline. That ear looked like an artifact.