Added DDD-awareness to improve-codebase-architecture

improve-codebase-architecture/DEEPENING.md

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+# Deepening
+
+How to deepen a cluster of shallow modules safely, given its dependencies.
+
+## Dependency categories
+
+When assessing a candidate for deepening, classify its dependencies. The category determines how the deepened module is tested.
+
+### 1. In-process
+
+Pure computation, in-memory state, no I/O. Always deepenable — just merge the modules and test directly.
+
+### 2. Local-substitutable
+
+Dependencies that have local test stand-ins (e.g. PGLite for Postgres, in-memory filesystem). Deepenable if the test substitute exists. The deepened module is tested with the local stand-in running in the test suite.
+
+### 3. Remote but owned (Ports & Adapters)
+
+Your own services across a network boundary (microservices, internal APIs). Define a port (interface) at the module boundary. The deep module owns the logic; the transport is injected. Tests use an in-memory adapter. Production uses the real HTTP/gRPC/queue adapter.
+
+Recommendation shape: *"Define a shared interface (port), implement an HTTP adapter for production and an in-memory adapter for testing, so the logic can be tested as one deep module even though it's deployed across a network boundary."*
+
+### 4. True external (Mock)
+
+Third-party services (Stripe, Twilio, etc.) you don't control. Mock at the boundary. The deepened module takes the external dependency as an injected port, and tests provide a mock implementation.
+
+## Testing strategy: replace, don't layer
+
+- Old unit tests on shallow modules are waste once boundary tests exist — delete them.
+- Write new tests at the deepened module's interface boundary.
+- Tests assert on observable outcomes through the public interface, not internal state.
+- Tests should survive internal refactors — they describe behavior, not implementation.