The `broadcast=1` path on adarWrite() emitted the 0x08 broadcast opcode
but setChipSelect() only asserts one device's CS line, so only the single
selected chip ever saw the frame. The opcode path has also never been
validated on silicon. Until a HIL test confirms multi-CS semantics, route
broadcast=1 through a unicast loop over all devices so caller intent
(all four take the write) is preserved and the dead opcode path becomes
unreachable. Logs a DIAG_WARN on entry for visibility.
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