Struct twins

tools/type_census.py --check-structs reads this file (TWINS_DOC). G62: layout twins without evidence stay apart.

## Twins: one ### <lhash> section per tier-2 duplicate row (plain or <lhash>:opaque) kept as separate types, with a names: line (the row's names, as a set) and a cause: line naming at least one address (8 hex digits or 0x…). A listed row does not count in dup_classes. An absent lhash, a name-set mismatch or an addressless cause counts in twins_stale.
## Dead kept: - Name — cause bullets; a listed canon name does not count in dead. A name that is not dead or no longer defined in the canonical headers counts in twins_stale.

Twins