feat(v7-d04): hybrid master-fp32 + bf16-fwd helpers #138

Job	Run time
Preset matrix (smoke, ubuntu) (2)	23s
Preset matrix (smoke, ubuntu) (4)	10s
Preset matrix (smoke, ubuntu) (3)	14s
Preset matrix (smoke, ubuntu) (1)	9s
Specialised + canvas smoke	17s
Mini-train matrix (macOS, nightly)	-1s
Matrix report	12s
	1m 24s

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