Phase 0 batch 3a. structinfo registration + fieldinfo per-field in
registerstruct (cgenutil.ww). Both relied on amalloc-zero for fields=nil
and totsize=0 (structinfo) and finext=nil (fieldinfo); MAP_ANON-zero
covers the same slots.
check.ww:842 (arenau64tos 24B scratch) deferred to #11.
cgenutil.ww:108 (mkvarargname runtime-N) deferred to #8.
Verified 132/132 + 995_self_rebuild byte-identity.