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/physics/test_exchange.py

@@ -0,0 +1,56 @@
+"""
+Physics 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 TestExchangePhysics:
+    """Test expected physical behavior of the exchange correction."""
+
+    @pytest.mark.physics
+    def test_large_correction_heavy_nucleus(self):
+        """For Z=80 (mercury), X >> 1 near threshold — can be > 20%.
+
+        This is the defining characteristic of the exchange correction.
+        """
+        ex = ExchangeCorrection(Z=80)
+        low_W = np.array([1.1])
+        result = ex(low_W)
+        assert result[0] > 1.0, f"Exchange must enhance spectrum at low energy for heavy Z: {result[0]}"
+
+    @pytest.mark.physics
+    def test_grows_as_energy_decreases(self):
+        """X(W) should increase as W → 1 (lower momentum → stronger exchange)."""
+        ex = ExchangeCorrection(Z=50)
+
+        low_W = np.array([1.2])
+        lower_W = np.array([1.05])
+
+        s_low = ex(low_W)[0]
+        s_lower = ex(lower_W)[0]
+
+        assert s_lower > s_low, f"Exchange must increase at lower energy: {s_low} → {s_lower}"
+
+    @pytest.mark.physics
+    def test_vanishes_at_high_energy(self):
+        """At W >> 1, the emitted electron is far from atomic electrons → X → 1."""
+        ex = ExchangeCorrection(Z=40)
+        high_W = np.array([3.0, 5.0])
+        result = ex(high_W)
+        assert abs(result[0] - 1.0) < 0.5, f"Exchange must approach unity at high energy: {result}"
+
+
+class TestExchangeErrorHandling:
+    """Test error handling for invalid inputs."""
+
+    @pytest.mark.api
+    def test_missing_Z_raises_error(self):
+        """Z=1 has no electrons to exchange with — should fail gracefully."""
+        with pytest.raises(ValueError, match="No exchange coefficients"):
+            ExchangeCorrection(Z=1)