> struct field alignment/padding isn't part of the C spec iirc

It's part of the ABI spec. It's true that C evolved in an ad hoc way and so the formal rigor got spread around to a bunch of different stakeholders. It's not true that C is a lawless wasteland where all behavior is subject to capricious and random whims, which is an attitude I see a lot in some communities.

People write low level software to deal with memory layout and alignment every day in C, have for fourty years, and aren't stopping any time soon.