Module missing 

MissingOr<T> — first-class summary placeholder for SSZ subtree substitution.

Two variants:

• Materialized(T) — the full value is present.
• Missing([u8; 32]) — only the precomputed hash_tree_root is known.

Hash invariant (the load-bearing property):

Missing(h).hash_tree_root::<D>()      == h
Materialized(t).hash_tree_root::<D>() == t.hash_tree_root::<D>()

No mix_in_selector is applied — that would defeat substitution.

Wire form (jar-specific extension; not standard SSZ):

• byte 0 = 0 + payload bytes (Materialized)
• byte 0 = 1 + 32 raw hash bytes (Missing)

Enums

MissingOr

Either a fully materialized value or a precomputed hash placeholder.