feat: add solver core skeleton

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

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+# Solver Core Skeleton Build/Test Report
+
+## Metadata
+- phase: solver-core-skeleton
+- scope: C++ skeleton classes only
+- status: passed
+
+## Commands Run
+
+```powershell
+python -m unittest discover -s scripts -p "test_*.py"
+```
+
+- exit_code: 0
+- summary: 98 Python Harness tests passed.
+
+```powershell
+python scripts/validate_workspace.py
+```
+
+- exit_code: 0
+- summary: CMake configure, MSVC Debug build, and full CTest suite passed.
+
+```powershell
+ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R solver_core_skeleton_integration_test
+```
+
+- exit_code: 0
+- summary: `solver_core_skeleton_integration_test` passed.
+
+## CTest Tests Added
+
+- `harness_smoke_test`
+- `core_diagnostic_test`
+- `core_ids_test`
+- `core_primitives_test`
+- `core_status_test`
+- `model_analysis_step_test`
+- `model_boundary_condition_test`
+- `model_domain_test`
+- `model_element_test`
+- `model_load_test`
+- `model_material_test`
+- `model_node_test`
+- `model_property_test`
+- `analysis_model_view_test`
+- `dof_manager_dof_key_test`
+- `dof_manager_numbering_test`
+- `analysis_state_vectors_test`
+- `analysis_flow_linear_static_analysis_test`
+- `analysis_flow_template_test`
+- `results_containers_test`
+- `solver_core_skeleton_integration_test`
+
+## Known Limitations
+
+- No element stiffness, residual, tangent, or stress recovery calculation is implemented.
+- No material law evaluation is implemented.
+- No sparse assembly implementation is implemented beyond `DofManager` sparse pattern ownership.
+- No linear solver backend is implemented.
+- No HDF5 writer or reader is implemented.
+- No Abaqus `.inp` parser is implemented.
+- No reference comparison against Abaqus CSV artifacts is implemented.
+- `LinearStaticAnalysis` currently prepares the analysis model, DOF map, and zero-valued state only.