docs: Seller Management capability documentation - Implemented/Planned/Future

Master entry-point doc (Seller-Management.md) consolidating everything
built across the prior 4 commits (ADR-011, domain models, Phase 1 UI,
UX review) plus the full roadmap, with every section explicitly
tagged Implemented / Planned / Future so nothing reads as built that
isn't.

Covers: Overview, Architecture & Hierarchy, Marketplace, Seller,
Roles & Permissions, Feature Flags, Bootstrap, Future API, Seller
Storefronts, Seller Branding, Seller Ownership, Checkout Modes,
Unified/Split Orders, Migration & Compatibility (why existing
marketplaces stay unchanged, with the concrete verification evidence
for each claim), Developer Notes, Builder Notes, Backend Notes.

Explicitly marked Future (not designed, no shape decided) rather than
documented as if real: the API surface, seller storefronts, checkout
modes, and the unified-vs-split-order decision - none of these have
any code or ADR behind them yet, unlike the typed models/feature flag/
Phase 1 UI which are genuinely Implemented.

Added a rollout-stage diagram (types+flag -> Phase 1 UI -> backend
decisions -> CRUD -> branding/storefronts -> checkout modes) showing
work stops after "Phase 1 UI" today. Linked as the entry point from
docs/architecture/foundation/README.md and docs/PROJECT_INDEX.md,
ahead of ADR-011/diagrams/domain-models/UX-review which stay as
detail references.

No code changed.

docs/architecture/foundation/README.md

@@ -49,6 +49,7 @@ It is a platform runtime that must support unlimited tenants from one Angular ap
 
 ### Seller Management (optional, in preparation — not built)
 
+- [Seller-Management.md](Seller-Management.md) — capability overview, start here
 - [ADR-011](adr/ADR-011-optional-seller-management-module.md) — decision record
 - [Seller-Management-Diagrams.md](Seller-Management-Diagrams.md) — hierarchy, bootstrap gate, type diagram
 - [Seller-Management-Domain-Models.md](Seller-Management-Domain-Models.md) — typed models, optional sellerId fields