feat(v7-c04): streaming safetensors checkpoint loader #135

Job	Run time
Specialised + canvas smoke	12s
Preset matrix (smoke, ubuntu) (2)	11s
Preset matrix (smoke, ubuntu) (1)	16s
Preset matrix (smoke, ubuntu) (4)	12s
Preset matrix (smoke, ubuntu) (3)	16s
Mini-train matrix (macOS, nightly)	0s
Matrix report	10s
	1m 17s

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