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_finite_size.py

@@ -0,0 +1,86 @@
+"""
+Physics tests for FiniteSizeL0 and ChargeDistributionU.
+
+Shared property tests in tests/common/test_property_tests.py.
+"""
+
+import numpy as np
+import pytest
+
+from beta_spectrum.components.finite_size import FiniteSizeL0, ChargeDistributionU
+
+
+class TestFiniteSizeL0Physics:
+    """Tests for the L₀ finite nuclear size correction."""
+
+    @pytest.mark.physics
+    def test_near_unity_for_light_nucleus(self):
+        """For Z=6 (carbon), L₀ should be very close to 1 — within a few percent."""
+        fs = FiniteSizeL0(Z=6, A=12)
+        W_test = np.array([2.0])
+        result = fs(W_test)
+
+        assert (
+            abs(result[0] - 1.0) < 0.05
+        ), f"L₀ for Z=6 should be near unity but got {result[0]}"
+
+    @pytest.mark.physics
+    def test_correction_increases_with_Z(self):
+        """Higher Z → larger αZ term in L₀ expansion → larger deviation from 1."""
+        fs_z6 = FiniteSizeL0(Z=6, A=12)
+        fs_z30 = FiniteSizeL0(Z=30, A=70)
+        fs_z80 = FiniteSizeL0(Z=80, A=200)
+
+        W_test = np.array([2.5])
+
+        dev_6 = abs(fs_z6(W_test)[0] - 1.0)
+        dev_30 = abs(fs_z30(W_test)[0] - 1.0)
+        dev_80 = abs(fs_z80(W_test)[0] - 1.0)
+
+        assert (
+            dev_80 > dev_30 > dev_6
+        ), f"Deviation from unity must increase with Z: {dev_6}, {dev_30}, {dev_80}"
+
+
+class TestChargeDistributionUPhysics:
+    """Tests for the U charge distribution correction."""
+
+    @pytest.mark.physics
+    def test_near_unity_for_light_nucleus(self):
+        """For Z=6, U is essentially 1."""
+        u = ChargeDistributionU(Z=6, A=12)
+        result = u(np.array([2.0]))
+        assert abs(result[0] - 1.0) < 0.01
+
+    @pytest.mark.physics
+    def test_always_above_unity(self):
+        """U = 1 + (1/5)(αZW R)² is always ≥ 1 since the squared term is positive."""
+        u = ChargeDistributionU(Z=20, A=40)
+        W_test = np.linspace(1.1, 5.0, 10)
+        result = u(W_test)
+        assert np.all(result >= 1.0), "U must be ≥ 1 for all W (squared term)"
+
+    @pytest.mark.physics
+    def test_increases_with_Z(self):
+        """Higher Z → larger U correction."""
+        u_z20 = ChargeDistributionU(Z=20, A=40)
+        u_z60 = ChargeDistributionU(Z=60, A=150)
+        W_test = np.array([3.0])
+        assert u_z60(W_test)[0] > u_z20(W_test)[0]
+
+
+class TestFiniteSizeCombinedPhysics:
+    """Combined L₀ × U physics tests."""
+
+    @pytest.mark.physics
+    def test_combined_near_unity_for_carbon(self):
+        """For carbon-12, both corrections are tiny. The product should be ~1."""
+        l0 = FiniteSizeL0(Z=6, A=12)
+        u = ChargeDistributionU(Z=6, A=12)
+
+        W_test = np.linspace(1.5, 4.0, 10)
+        combined = l0(W_test) * u(W_test)
+
+        assert np.all(
+            np.abs(combined - 1.0) < 0.05
+        ), f"Combined finite-size for carbon should be near unity: {combined}"