M5 MacBook Pro (10-core GPU, 16GB) reproduces the head_dim=56 silent fallback
and shows the fix matters far MORE on narrow GPUs: padding 56->64 gives 4.9x
(1,16,4096), 5.1x (1,16,2048), 3.2x (4096,2,64). Raw fused-vs-unfused at
4096/fp32 is 0.14x on M5 vs ~0.53x on M3 Ultra — an M5 laptop matches an M1
Ultra on fused attention but is ~4x slower unfused. This reframes the fix as a
laptop optimization, not a big-iron one.
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