ssa-lowering

Internal skill: the reasoning front-end that lowers one-or-more change requests into a partitioned set of Missions — one owning Mission per spec-node (SSA) — applying the Oracle (should we?) and Architect (where/barrier?) lenses, resolving same-node contention into ordered versioned-RAW edges, and lowering only the frontier deeply; run by the conductor during intake/Explore, not triggered by users directly.