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/integration/__init__.py

@@ -0,0 +1,8 @@
+"""
+Integration tests for the beta-spectrum calculator.
+
+These tests verify end-to-end workflows that span multiple components:
+- Config → spectrum → convolve → export
+- Multi-branch → weighted sum → export
+- Detector response integration
+"""