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/quality/test_api_exchange.py

@@ -0,0 +1,47 @@
+"""
+Code quality / API tests for ExchangeCorrection.
+
+Shared property tests in tests/common/test_property_tests.py.
+"""
+
+import numpy as np
+import pytest
+
+from beta_spectrum.components.exchange import ExchangeCorrection
+
+
+class TestExchangeAPI:
+    @pytest.mark.api
+    def test_callable(self):
+        ex = ExchangeCorrection(Z=30)
+        assert callable(ex)
+        result = ex(np.array([2.0]))
+        assert isinstance(result, np.ndarray)
+
+class TestExchangeProperties:
+    @pytest.mark.property
+    @pytest.mark.parametrize("W", [
+        [2.0],
+        np.linspace(1.01, 6.0, 50),
+    ])
+    def test_positive(self, W):
+        ex = ExchangeCorrection(Z=92)
+        from tests.common.test_property_tests import _test_positive_values
+        _test_positive_values(ex, W, component_name="ExchangeCorrection")
+
+    @pytest.mark.quality
+    @pytest.mark.parametrize("n", [1, 5, 100])
+    def test_shape(self, n):
+        ex = ExchangeCorrection(Z=30)
+        W = np.linspace(1.1, 3.0, n)
+        from tests.common.test_property_tests import _test_output_shape_preserved
+        _test_output_shape_preserved(ex, W, component_name="ExchangeCorrection")
+
+    @pytest.mark.quality
+    @pytest.mark.parametrize("W", [
+        np.linspace(1.01, 6.0, 50),
+    ])
+    def test_no_nan(self, W):
+        ex = ExchangeCorrection(Z=92)
+        from tests.common.test_property_tests import _test_no_nan_across_range
+        _test_no_nan_across_range(ex, W, component_name="ExchangeCorrection")