fix(portable): load BF16 weights by gating arena-only copies off the … #163

Job	Run time
pie-server (driver-dummy) (macos-latest)	1m 26s
pie-server (driver-dummy) (ubuntu-latest)	1m 5s
pie-server (driver-portable + metal)	2m 52s
cargo check / test (workspace)	1m 15s
pie-server (driver-portable)	2m 2s
pie-server (driver-cuda)	0s
	8m 40s

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