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/PROJECT_INDEX.md

@@ -14,7 +14,7 @@ Every tenant has three surfaces on this one codebase:
 ## System overview
 
 - **Architecture**: `Component (container) → Facade → Domain Service → Repository/Provider (DI token, swappable mock↔API) → Mock | API`. Enforced by `npm run arch:check` (import boundaries + circular deps), not just convention. Full detail: [ARCHITECTURE.md](ARCHITECTURE.md), governance docs at `docs/architecture/foundation/**` (11 ADRs + 9 standards docs).
-- **Seller Management** (optional, not built): typed foundation only — `modules.sellerManagement.enabled` gate on `BootstrapConfig`, disabled by default, zero effect on existing marketplaces. See ADR-011 and `docs/architecture/foundation/Seller-Management-Diagrams.md`.
+- **Seller Management** (optional, not built): typed foundation + Phase 1 Backoffice placeholder UI only — `modules.sellerManagement.enabled` gate on `BootstrapConfig`, disabled by default, zero effect on existing marketplaces. Full capability doc: `docs/architecture/foundation/Seller-Management.md`.
 - **State**: Signals-based facades everywhere, no NgRx (ADR-007).
 - **Rendering**: Bootstrap JSON → Section Engine → Page Renderer → Widget Host → registered widget component (ADR-005). 100% lazy-loaded routes.
 - **Theming**: CSS custom properties per tenant, 3 theme stylesheets, never hardcoded hex in a component (ADR-008). Design system spec: [`DESIGN.md`](../DESIGN.md) (root of repo).