Next.js 15 App Router Enterprise Architecture & Route Group Blueprint

You are a Principal Next.js Framework Architect and Vercel Ecosystem Specialist. I need to design an enterprise-grade file and folder architecture for a complex web application using the Next.js 15 App Router. Application Feature Scope & User Roles: B2B SaaS platform with public SEO marketing pages, documentation hub, OAuth2 login/signup flows, and an enterprise multi-tenant team dashboard with role-based workspace switching. Key Routing Scenarios (Deep-link Modals, Auth Gates, Public vs. App Layouts): 1. Marketing pages and docs share a sleek dark footer, while dashboard has a persistent sidebar. 2. Project creation modal can be opened via URL (`/projects/new`) as a modal or viewed full-screen. 3. Instant optimistic updates on project cards. Data Layer & Server Action Requirements: Prisma ORM with Server Actions, Zod validation, and fine-grained `revalidateTag` caching. Generate an exhaustive App Router Architecture Blueprint: 1. Complete Directory Tree (`src/app/`): Provide a complete file tree using route groups `(public)`, `(auth)`, `(dashboard)`, parallel slots (`@modal`), and intercepting routes (`(.)photos/<id>`), showing where `layout.tsx`, `page.tsx`, `loading.tsx`, `error.tsx`, and `not-found.tsx` reside. 2. Root Layout vs Sub-Layout Optimization: Explain how to prevent unmounting and re-rendering of global navigation when navigating between dashboard sub-routes. 3. Deep-Linking Intercepted Modal Implementation: Detail how to configure intercepting routes so clicking an item opens an overlay modal with a distinct URL, but refreshing the page directly renders a full standalone page. 4. Server Action & Mutation Colocation: Best practices for collocating Server Actions with routes while preventing unintended data exposure.