refactor: move solver skeleton implementations to cpp

docs/build-test-reports/solver-core-skeleton.md

@@ -7,12 +7,19 @@
 
 ## Commands Run
 
+```powershell
+python -m unittest scripts.test_header_declaration_only
+```
+
+- exit_code: 0
+- summary: Solver headers contain declarations only; function bodies are implemented in `.cpp` files.
+
 ```powershell
 python -m unittest discover -s scripts -p "test_*.py"
 ```
 
 - exit_code: 0
-- summary: 98 Python Harness tests passed.
+- summary: 99 Python Harness tests passed.
 
 ```powershell
 python scripts/validate_workspace.py
@@ -52,6 +59,15 @@ ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R solver_core_sk
 - `results_containers_test`
 - `solver_core_skeleton_integration_test`
 
+## Structural Tests Added
+
+- `scripts.test_header_declaration_only`
+
+## Build Structure
+
+- `fesa_core` now builds as a static library from `src/fesa/**/*.cpp`.
+- Solver headers under `src/fesa/**/*.hpp` declare functions only; method bodies live in matching `.cpp` translation units.
+
 ## Known Limitations
 
 - No element stiffness, residual, tangent, or stress recovery calculation is implemented.