test: create new test architecture with shared property/API tests

- Add tests/common/ with parametrized property tests (positivity, shape, NaN, stability)
- Add tests/physics/ with unique physics behavior tests for 6 correction components
- Add tests/quality/ with API contract tests for 6 correction components
- Add pytest markers in pyproject.toml: property, physics, api, quality, integration
- New tests: 94 tests passing (physics + quality + common helpers)
- Old tests in tests/ root still present — migration in progress
- Total: 261 tests collected (230 old + 94 new - 63 duplicates removed)

The shared helpers eliminate 18 duplicate test patterns across correction
components. Each component now has:
  - tests/physics/test_<name>.py — unique physics behavior
  - tests/quality/test_api_<name>.py — API contract + shared property checks
  - tests/common/test_property_tests.py — shared helpers (imported, not duplicated)

tests/common/test_api_tests.py

@@ -0,0 +1,89 @@
+"""
+Shared API contract tests for all SpectrumComponent subclasses.
+
+These tests verify that every component adheres to the SpectrumComponent
+interface contract. They are parameterized to run against all component
+classes automatically.
+
+## Contract requirements
+
+1. `__call__(W) -> np.ndarray` — the component is callable with W array
+2. `__init__` accepts `logger: Optional[Logger] = None`
+3. `__repr__` returns a non-empty string
+4. Subclasses implement `__call__` (abstract method)
+
+## Usage
+
+Each component test file imports these helpers:
+
+```python
+import pytest
+from beta_spectrum.components.fermi import FermiFunction
+from tests.common.test_api_tests import test_component_is_callable
+
+class TestFermiAPI:
+    @pytest.mark.api
+    def test_callable(self):
+        ff = FermiFunction(Z=20, A=40)
+        test_component_is_callable(ff, "FermiFunction")
+```
+"""
+
+import numpy as np
+import pytest
+
+
+@pytest.mark.api
+def _test_component_is_callable(component, component_name="Component"):
+    """Component must be callable with array input."""
+    assert callable(component), (
+        f"{component_name}: component is not callable"
+    )
+
+    W = np.array([1.5, 2.0, 3.0])
+    result = component(W)
+
+    assert isinstance(result, np.ndarray), (
+        f"{component_name}: __call__ returned {type(result)}, "
+        f"expected np.ndarray"
+    )
+    assert result.shape == W.shape, (
+        f"{component_name}: shape mismatch {W.shape} → {result.shape}"
+    )
+
+
+@pytest.mark.api
+def _test_component_repr(component, component_name="Component"):
+    """Component must have a non-empty repr."""
+    repr_str = repr(component)
+    assert isinstance(repr_str, str), (
+        f"{component_name}: repr returned {type(repr_str)}"
+    )
+    assert len(repr_str) > 0, (
+        f"{component_name}: repr is empty string"
+    )
+    # repr should contain the class name
+    assert component_name in repr_str or component.__class__.__name__ in repr_str, (
+        f"{component_name}: repr should contain class name. Got: {repr_str}"
+    )
+
+
+@pytest.mark.api
+def _test_component_list_input(component, component_name="Component"):
+    """Component must handle Python list input (numpy converts it)."""
+    result = component([1.5, 2.0, 3.0])
+    assert isinstance(result, np.ndarray), (
+        f"{component_name}: list input returned {type(result)}"
+    )
+
+
+@pytest.mark.api
+def _test_component_empty_input(component, component_name="Component"):
+    """Component must handle empty array input gracefully."""
+    result = component(np.array([]))
+    assert isinstance(result, np.ndarray), (
+        f"{component_name}: empty input returned {type(result)}"
+    )
+    assert result.shape == (0,), (
+        f"{component_name}: empty input should return empty array"
+    )