v0.6.20: SM120 hybrid fusion and FFN activation lifetime

Adds strict BF16 FFN/QKV adapter fusion for the SM120 hybrid four-step profile and releases dead FFN modulation activations earlier.

A same-instance RTX 5090 comparison reduced denoising by about 2.1% and peak denoiser allocation by 2.14 GiB with identical complete MP4 outputs. Earlier activation release saved another 0.99 GiB without a measured speed gain. A previously failing 1536×864 fifteen-second case completed in 431.6 seconds with a 27.92 GiB denoiser peak. A 2048×2048 five-second case still exceeded capacity; these results do not qualify every shape or imply general video quality improvement.

Validation: 46 focused tests from committed source and the installed wheel, privacy checks, bilingual documentation build, and exact-commit CI/Pages. Full videos, multiple frames and real browser playback were inspected. See the bilingual hardware profile and release notes for scope and remaining limits.