effective_denoise_steps computed ceil(steps * strength) (steps - int(steps - steps*strength)),
but diffusers get_timesteps denoises init_timestep = min(int(num_inference_steps * strength),
num_inference_steps), i.e. the floored product. The two differ by one whenever the product is
fractional, and that flips the auto FBCache decision in the (19, 20) band: a strength-0.7
28-step img2img denoises int(19.6) = 19 real steps (below FBCACHE_MIN_STEPS = 20) but the old
formula returned 20 and engaged FBCache on that short trajectory, exactly the quality hit the
auto policy exists to avoid. Return min(int(steps * strength), steps) to match diffusers, and
fix the two tests that replayed the old formula.
Also honor _default_threads' documented fallback: (os.cpu_count() or 8) // 2 yields 4 when the
count is unknown, contradicting the docstring's 'falls back to 8'. Return 8 in that case.
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