feat: add domain model foundation

phases/domain-model-foundation/step3.md

@@ -0,0 +1,63 @@
+# Step 3: node-and-domain-storage
+
+## Read First
+
+Read these files before editing:
+
+- `/AGENTS.md`
+- `/docs/AGENT_RULES.md`
+- `/docs/ADR.md`
+- `/docs/ARCHITECTURE.md`
+- `/docs/implementation-plans/domain-model-foundation-implementation-plan.md`
+- `/include/fesa/core/ModelTypes.hpp`
+- `/CMakeLists.txt`
+
+## Task
+
+Implement the first useful `Node` and `Domain` slice.
+
+Allowed files:
+
+- Create `/include/fesa/core/Node.hpp`
+- Create `/include/fesa/core/Domain.hpp`
+- Create `/src/core/Domain.cpp`
+- Create or modify `/tests/core/domain_storage_test.cpp`
+- Modify `/CMakeLists.txt` to compile the source and register the test
+
+Required behavior:
+
+- `Node` stores `NodeId` and three coordinates as `double`.
+- `Domain::addNode(Node)` inserts a node.
+- Duplicate node ids throw `std::invalid_argument`.
+- `Domain::findNode(NodeId)` returns a pointer or `nullptr`.
+- `Domain::node(NodeId)` returns a const reference or throws `std::out_of_range`.
+- `Domain::nodeCount()` returns the stored node count.
+- `Domain` must not expose mutable node references.
+
+## Tests To Write First
+
+Write failing tests before creating production headers/sources:
+
+- Add and retrieve one node by id.
+- Missing node returns `nullptr` from `findNode`.
+- Missing node throws from `node`.
+- Duplicate node id throws.
+
+Run the targeted CTest and verify failure due to missing API. Then implement the minimal code.
+
+## Acceptance Criteria
+
+Run:
+
+```powershell
+cmake --build build/msvc-debug --config Debug
+ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R domain
+python scripts/validate_workspace.py
+```
+
+Update `/phases/domain-model-foundation/index.json` step 3 with `completed`, `error`, or `blocked`.
+
+## Do Not
+
+- Do not add elements, materials, properties, sets, loads, or steps yet.
+- Do not store equation ids, displacement vectors, residuals, reactions, or solver state.