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/common/__init__.py

@@ -0,0 +1,18 @@
+"""
+Shared test utilities for the beta-spectrum test suite.
+
+This module provides parametrized test helpers that eliminate duplicate
+test code across correction components. Instead of each component
+writing its own `test_positive`, `test_output_shape`, `test_no_nan`
+functions, they import and use these shared helpers.
+
+## Design principles
+
+1. **Parametrized, not duplicated** — one `test_positive_everywhere` helper
+   covers all components via pytest.mark.parametrize.
+2. **Fail-fast** — if a helper fails, the error message identifies the
+   component and the violated property.
+3. **Physics-aware** — helpers understand that some components (like
+   phase_space) return zero at threshold, so "positive" means "non-negative
+   with at least one strictly positive value."
+"""