Use a three-way semantic merge spec before replacing a mature simulation subsystem

When a broad existing subsystem and a cleaner isolated prototype overlap, document the migration like a semantic three-way merge. For every capability and concrete function, record behavior in A, behavior in B, required result in C, what is kept, what is rejected, and the parity proof required before deletion. Include load-bearing neighboring mechanisms that neither branch owns, known prototype defects that must not be normalized as intended behavior, explicit no-ship conditions, and a work-package protocol requiring builders to name reused seams before coding. This prevents a cleaner abstraction from silently deleting compute costs, lifecycle actions, live adapters, or reachable emergent behavior.