wip(soa): Wave-A draft — SoaContainer LE repr(C) container type [un-reviewed]

Sonnet Wave-A agent draft (surreal task 02): SoaContainerHeader<N>
#[repr(C, align(8))] pointer-free LE container + golden-byte/roundtrip tests,
built on hpc::soa (#156). NOT yet Opus-reviewed/compiled. Pending decisions:
add bytemuck + derive Pod/Zeroable (kill manual unsafe), rename → SoaContainer,
explicit odd-N pad. Committed as WIP per stop-hook; does not yet compile
(bytemuck not in deps).

Author: claude