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docs/architecture/core/CORE_DATA_FLOW.md

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+# Core Data Flow (Strict)
+
+This document defines the **Core** data flow rules and boundaries.
+
+Core scope:
+
+- `core/**`
+
+Core does not know:
+
+- HTTP
+- Next.js
+- databases
+- DTOs
+- UI models
+
+## 1) Layers inside Core
+
+Core contains two inner layers:
+
+- Domain
+- Application
+
+### 1.1 Domain
+
+Domain is business truth.
+
+Allowed:
+
+- Entities
+- Value Objects
+- Domain Services
+
+Forbidden:
+
+- DTOs
+- frameworks
+- IO
+
+See [`docs/architecture/core/DOMAIN_OBJECTS.md`](docs/architecture/core/DOMAIN_OBJECTS.md:1).
+
+### 1.2 Application
+
+Application coordinates business intents.
+
+Allowed:
+
+- Use Cases (commands and queries)
+- Application-level ports (repository ports, gateways)
+
+Forbidden:
+
+- HTTP
+- persistence implementations
+- frontend models
+
+## 2) Core I/O boundary
+
+All communication across the Core boundary occurs through **ports**.
+
+Rules:
+
+- Port interfaces live in Core.
+- Implementations live outside Core.
+
+## 3) Core data types (strict)
+
+- Use Case inputs are plain data and/or domain types.
+- Use Case outputs are plain data and/or domain types.
+
+Core MUST NOT define HTTP DTOs.
+
+## 4) Canonical flow
+
+```text
+Delivery App (HTTP or Website)
+  ↓
+Core Application (Use Case)
+  ↓
+Core Domain (Entities, Value Objects)
+  ↓
+Ports (repository, gateway)
+  ↓
+Adapter implementation (outside Core)
+```
+
+## 5) Non-negotiable rules
+
+1. Core is framework-agnostic.
+2. DTOs do not enter Core.
+3. Core defines ports; outer layers implement them.
+