test: complete migration to structured test suite

Phase 1: Removed 6 duplicate old test files (fermi, finite_size, screening,
exchange, radiative, phase_space) — their tests are now in physics/ and quality/
using shared helpers from common/.

Phase 2: Migrated remaining 7 old test files:
- test_spectrum.py → integration/test_spectrum_pipeline.py (integration tests)
- test_fitter.py → physics/test_fitter.py (fit quality tests)
- test_cw_extractor.py → physics/test_cw_extractor.py (extraction accuracy)
- test_multi_branch.py → physics/test_multi_branch.py (branching physics)
- test_detector_response.py → physics/ + quality/ (split into physics + API)
- test_nuclear_data.py → quality/test_nuclear_data.py (parsing/API)
- test_logging_utils.py → quality/test_logging_utils.py (API tests)

Removed 12 old flat files, created 4 new files.
Total: 251 tests passing (6 skipped), down from 230 → 251 (net +21
from better coverage, not duplicates).

Test structure:
- tests/common/ — shared property/API helpers (eliminates ~18 duplicates)
- tests/physics/ — 10 files, unique physics behavior tests
- tests/quality/ — 8 files, API contract + parsing tests
- tests/integration/ — 1 file, end-to-end pipeline tests

tests/quality/test_nuclear_data.py

@@ -0,0 +1,322 @@
+"""Tests for nuclear_data module (paceENSDF integration and JSON input)."""
+
+import json
+import tempfile
+from pathlib import Path
+from typing import Any, Dict
+
+import pytest
+
+# ruff: noqa: E402, ANN201, ANN001
+from beta_spectrum.nuclear_data import (  # noqa: E402
+    BranchInfo,
+    DecayInfo,
+    create_config_from_source,
+    decay_info_to_config,
+    get_decay_info_from_paceENSDF,
+    json_to_config,
+    load_json_input,
+    validate_json_input,
+    _parse_nuclide_symbol,
+)
+from beta_spectrum.spectrum import SpectrumConfig
+
+
+def _has_paceENSDF() -> bool:  # noqa: ANN201
+    """Check if paceENSDF is available."""
+    try:
+        import paceENSDF  # noqa: F401
+
+        return True
+    except ImportError:
+        return False
+
+
+# ---------------------------------------------------------------------------
+# _parse_nuclide_symbol
+# ---------------------------------------------------------------------------
+
+
+class TestParseNuclideSymbol:
+    """Test nuclide symbol parsing."""
+
+    def test_tc99(self):
+        element, Z, A = _parse_nuclide_symbol("Tc99")
+        assert element == "Tc"
+        assert Z == 43
+        assert A == 99
+
+    def test_co60(self):
+        element, Z, A = _parse_nuclide_symbol("Co60")
+        assert element == "Co"
+        assert Z == 27
+        assert A == 60
+
+    def test_he3(self):
+        element, Z, A = _parse_nuclide_symbol("He3")
+        assert element == "He"
+        assert Z == 2
+        assert A == 3
+
+    def test_invalid_symbol(self):
+        with pytest.raises(ValueError, match="Cannot parse nuclide symbol"):
+            _parse_nuclide_symbol("99Tc")
+
+    def test_unknown_element(self):
+        with pytest.raises(ValueError, match="Unknown element symbol"):
+            _parse_nuclide_symbol("Xx99")
+
+
+# ---------------------------------------------------------------------------
+# validate_json_input
+# ---------------------------------------------------------------------------
+
+
+class TestValidateJsonInput:
+    """Test JSON input validation."""
+
+    def test_valid_input(self):
+        data = {
+            "endpoint_MeV": 0.5,
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+        }
+        validate_json_input(data)  # should not raise
+
+    def test_missing_endpoint(self):
+        data = {"Z_parent": 43, "Z_daughter": 44, "A_number": 99}
+        with pytest.raises(ValueError, match="Missing required field"):
+            validate_json_input(data)
+
+    def test_negative_endpoint(self):
+        data = {
+            "endpoint_MeV": -0.5,
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+        }
+        with pytest.raises(ValueError, match="endpoint_MeV must be positive"):
+            validate_json_input(data)
+
+    def test_invalid_transition_type(self):
+        data = {
+            "endpoint_MeV": 0.5,
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+            "transition_type": "invalid",
+        }
+        with pytest.raises(ValueError, match="Invalid transition_type"):
+            validate_json_input(data)
+
+
+# ---------------------------------------------------------------------------
+# load_json_input / json_to_config
+# ---------------------------------------------------------------------------
+
+
+class TestJsonInput:
+    """Test JSON file loading and conversion."""
+
+    def _write_json(self, data: Dict[str, Any]) -> Path:
+        """Write data to a temp JSON file."""
+        tmp = tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False)
+        json.dump(data, tmp)
+        tmp.close()
+        return Path(tmp.name)
+
+    def test_load_and_convert(self):
+        data = {
+            "parent_symbol": "Tc99",
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+            "endpoint_MeV": 0.294,
+            "transition_type": "F2",
+            "e_step_MeV": 0.001,
+        }
+        path = self._write_json(data)
+        try:
+            loaded = load_json_input(str(path))
+            config = json_to_config(loaded)
+            assert isinstance(config, SpectrumConfig)
+            assert config.Z_parent == 43
+            assert config.Z_daughter == 44
+            assert config.A_number == 99
+            assert abs(config.endpoint_MeV - 0.294) < 1e-6
+            assert config.transition_type == "F2"
+            assert config.e_step_MeV == 0.001
+        finally:
+            path.unlink()
+
+    def test_defaults_filled(self):
+        data = {
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+            "endpoint_MeV": 0.294,
+        }
+        path = self._write_json(data)
+        try:
+            loaded = load_json_input(str(path))
+            # Defaults should be filled in
+            assert loaded["detector_model"] == "gaussian"
+            assert loaded["detector_sigma_a_keV"] == 1.0
+        finally:
+            path.unlink()
+
+    def test_file_not_found(self):
+        with pytest.raises(FileNotFoundError):
+            load_json_input("/nonexistent/file.json")
+
+
+# ---------------------------------------------------------------------------
+# decay_info_to_config
+# ---------------------------------------------------------------------------
+
+
+class TestDecayInfoToConfig:
+    """Test conversion from DecayInfo to SpectrumConfig."""
+
+    def test_basic_conversion(self):
+        info = DecayInfo(
+            parent_symbol="Tc99",
+            Z_parent=43,
+            Z_daughter=44,
+            A_number=99,
+            endpoint_keV=294000.0,
+            endpoint_MeV=0.294,
+            transition_type="A",
+            forbiddenness_code="0A",
+            parent_J=4.5,
+            parent_pi=1,
+            half_life="y",
+            branches=[],
+        )
+        config = decay_info_to_config(info)
+        assert isinstance(config, SpectrumConfig)
+        assert config.Z_parent == 43
+        assert config.Z_daughter == 44
+        assert config.A_number == 99
+        assert abs(config.endpoint_MeV - 0.294) < 1e-6
+        assert config.transition_type == "A"
+
+    def test_with_detector_response(self):
+        info = DecayInfo(
+            parent_symbol="Co60",
+            Z_parent=27,
+            Z_daughter=28,
+            A_number=60,
+            endpoint_keV=317860.0,
+            endpoint_MeV=0.31786,
+            transition_type="A",
+            forbiddenness_code="1A",
+            parent_J=0.0,
+            parent_pi=1,
+            half_life="y",
+            branches=[],
+        )
+        config = decay_info_to_config(
+            info, use_detector_response=True, e_step_MeV=0.005
+        )
+        assert config.use_detector_response is True
+        assert config.e_step_MeV == 0.005
+
+
+# ---------------------------------------------------------------------------
+# get_decay_info_from_paceENSDF
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.skipif(not _has_paceENSDF(), reason="paceENSDF not installed")
+class TestPaceENSDFIntegration:
+    """Test paceENSDF data retrieval."""
+
+    def test_tc99_beta_minus(self):
+        info = get_decay_info_from_paceENSDF("Tc99", "beta_minus")
+        assert isinstance(info, DecayInfo)
+        assert info.parent_symbol == "Tc99"
+        assert info.Z_parent == 43
+        assert info.Z_daughter == 44
+        assert info.A_number == 99
+        assert info.endpoint_MeV > 0
+        assert len(info.branches) > 0
+
+    def test_co60_beta_minus(self):
+        info = get_decay_info_from_paceENSDF("Co60", "beta_minus")
+        assert isinstance(info, DecayInfo)
+        assert info.parent_symbol == "Co60"
+        assert info.Z_parent == 27
+        assert info.Z_daughter == 28
+        assert info.A_number == 60
+        assert info.endpoint_MeV > 0
+
+    def test_branch_info(self):
+        info = get_decay_info_from_paceENSDF("Tc99", "beta_minus")
+        for branch in info.branches:
+            assert isinstance(branch, BranchInfo)
+            assert branch.level_index >= 0
+            assert branch.level_energy_keV >= 0
+            assert 0 < branch.intensity <= 1.0
+            if branch.log_ft is not None:
+                assert branch.log_ft > 0
+
+    def test_tc99_intensity_values(self):
+        """Tc99 intensities from ENSDF: GS=99.9984%, 89.5keV=0.0016%."""
+        info = get_decay_info_from_paceENSDF("Tc99", "beta_minus")
+        gs = next(b for b in info.branches if b.level_energy_keV == 0.0)
+        ex = next(b for b in info.branches if b.level_energy_keV == 89.5)
+        assert abs(gs.intensity - 0.999984) < 1e-6
+        assert abs(ex.intensity - 0.000016) < 1e-6
+
+    def test_invalid_nuclide(self):
+        with pytest.raises(ValueError, match="Unknown element symbol"):
+            get_decay_info_from_paceENSDF("Xx99", "beta_minus")
+
+    def test_no_decay_data(self):
+        # Pb208 is stable, no beta-minus decay data
+        with pytest.raises(ValueError, match="No BM decay data found"):
+            get_decay_info_from_paceENSDF("Pb208", "beta_minus")
+
+
+# ---------------------------------------------------------------------------
+# create_config_from_source
+# ---------------------------------------------------------------------------
+
+
+class TestCreateConfigFromSource:
+    """Test the unified config creation interface."""
+
+    def _write_json(self, data: Dict[str, Any]) -> Path:
+        tmp = tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False)
+        json.dump(data, tmp)
+        tmp.close()
+        return Path(tmp.name)
+
+    def test_json_source(self):
+        data = {
+            "Z_parent": 43,
+            "Z_daughter": 44,
+            "A_number": 99,
+            "endpoint_MeV": 0.294,
+        }
+        path = self._write_json(data)
+        try:
+            config = create_config_from_source("json", json_path=str(path))
+            assert isinstance(config, SpectrumConfig)
+            assert config.Z_parent == 43
+        finally:
+            path.unlink()
+
+    def test_unknown_source(self):
+        with pytest.raises(ValueError, match="Unknown source"):
+            create_config_from_source("unknown", nuclide="Tc99")
+
+    def test_paceENSDF_requires_nuclide(self):
+        with pytest.raises(ValueError, match="nuclide parameter required"):
+            create_config_from_source("paceENSDF")
+
+    def test_json_requires_path(self):
+        with pytest.raises(ValueError, match="json_path parameter required"):
+            create_config_from_source("json")