feat: add analysis model objects

phases/analysis-model-objects/step2.md

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+# Step 2: element-base-interface
+
+## Read First
+
+Read these files before editing:
+
+- `/AGENTS.md`
+- `/docs/AGENT_RULES.md`
+- `/docs/ARCHITECTURE.md`
+- `/docs/implementation-plans/analysis-model-objects-implementation-plan.md`
+- `/include/fesa/core/ElementDefinition.hpp`
+- `/CMakeLists.txt`
+
+## Task
+
+Add the abstract element model interface.
+
+Allowed files:
+
+- Create `/include/fesa/element/Element.hpp`
+- Create `/tests/element/element_base_test.cpp`
+- Modify `/tests/core/domain_bootstrap_test.cpp` only if the current test executable remains the single test runner
+- Modify `/CMakeLists.txt`
+
+Required interface:
+
+- virtual destructor.
+- `ElementId id() const noexcept`.
+- `ElementType type() const noexcept`.
+- `std::size_t nodeCount() const noexcept`.
+- `std::span` is not allowed because this project targets C++17.
+- Use const reference return for connectivity data.
+- no stiffness, residual, stress, state, or equation API in this phase.
+
+## Tests To Write First
+
+Write `/tests/element/element_base_test.cpp` before production changes. Use a small test subclass to prove:
+
+- base pointer dispatch works.
+- id/type/connectivity/property id are readable through base API.
+- deleting through `std::unique_ptr<Element>` is valid.
+
+Run targeted build and confirm RED because `Element.hpp` is missing.
+
+## Acceptance Criteria
+
+Run:
+
+```powershell
+python scripts/validate_workspace.py
+ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model-object
+```
+
+Update step 2 status.
+
+## Do Not
+
+- Do not implement MITC4 stiffness or numerical element behavior.