verify: emblem_v1 fixture (6 models) + Tier-2 gates for sd21/sd35/cascade

tier2: add sdxl + pixart end-to-end golden PNGs

pixart: add adaln.embedded_timestep tap

add still_life_v1 fixture goldens (2nd prompt, 6 models)

pixart: dit.block0 with cross-attention pad mask

pixart+sd35: add t5.hidden (masked T5 caption) goldens

cascade: stage_c.block0 (fine corr 1.0) replaces coarse stage_c.out

tier2: add sd15 golden.png (end-to-end regression gate)

goldens: add animatediff motion.block0 (motion-module tap)

goldens: add stage_c.out (cascade Stage-C denoiser, coarse gate)

goldens: add dit.block0 (pixart) + mmdit.block0 (sd35) transformer-core taps

goldens: add unet.out (sd15/sdxl) + unet.mid (sdxl) denoiser taps

goldens: 6 models (sd15/sd21/sdxl/pixart/stable-cascade/sd35-medium) @ diffusers 0.38

goldens: sd15 + sd21 + sdxl(CLIP-L pad-token fix) @ diffusers 0.38

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initial commit