Adds 'Capability structure for the wiki engine (reuse-surface-aligned)' to
UseCaseCatalog.md (no UC renumbered): a small always-on engine core (EC-1..EC-5),
ten typed per-shard-activatable extensions (X-OVERLAY/AUTHZ/VIEW/STRUCT/ADDR/COMP/
PROV/COLLAB/FED/ATT) each mapped to a reuse-surface capability with activation
defaults, and a conflict-mediation map turning conflicting requirements into
mechanisms. Bridges demand -> the engine typed-extension model (T5). Marks T2 done.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>