Documents the decision behind the typed contracts added in the
previous commit: Seller Management is an optional platform capability
module (Platform -> Marketplace -> Seller, 0..N per marketplace), not
a second tenancy tier. Backend resolves seller scope the same way it
already resolves tenant (ADR-001); frontend never resolves it itself.
Gated by one typed flag (modules.sellerManagement.enabled), same
capability-guard discipline as ADR-009, defaulting to disabled/absent
so existing marketplaces are byte-identical.
Explicitly scopes out UI, backend, and business logic as future work
requiring its own ADR/implementation pass once the module is actually
built out.
Added companion diagrams (Seller-Management-Diagrams.md): hierarchy,
bootstrap module-gate flow, and the type-contract class diagram.
Registered ADR-011 in the foundation README's ADR index.
