The 'auto' attention-backend resolver picked 'sage' whenever the
sageattention package was merely importable. But diffusers requires a
specific version with compiled CUDA kernels (0.38 needs >=2.1.1); an old
v1 wheel is importable yet rejected, which would silently drop to plain
SDPA instead of Flash. Gate on diffusers' own _CAN_USE_SAGE_ATTN (with an
is_sageattention_version fallback) so 'auto' chooses sage only when it can
actually dispatch, else flash, else SDPA.

SageAttention 2.2.0 is now built from source in the venv (CUDA kernels,
TORCH_CUDA_ARCH_LIST=8.6), so 'auto' resolves to sage on this host.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce