Clean up spine math utilities

[francois-drielsma]

Summary

This PR cleans up spine.math, expands focused test coverage, and fixes a few functional issues found during review.

Functional Changes

• Fixes closest_pair(..., iterative=True): the old loop found a closest point in the opposite set but did not switch sides for the next iteration, so the iterative path was not truly alternating between sets.
• Reworks union_find to use a parent-array implementation with path compression. Group membership is preserved, while groups keys now match compact returned labels when return_inverse=True.
• Adds validation for invalid inputs:
  • mismatched label lengths in contingency_table and adjusted_rand_score
  • non-positive KNN k
  • negative radius / epsilon and non-positive DBSCAN min_samples
  • undersampled PCA inputs
• Fixes call sites that passed "recursive" positionally into boolean iterative.

Cleanup

• Replaces Numba-specific annotations in signatures with Python / NumPy annotations.
• Uses named metric constants instead of repeated np.int64(...) casts.
• Cleans docstring shape notation and typo-level documentation issues.
• Adds targeted ignores for valid Numba jitclass decorators that Pylance cannot type correctly.

Coverage

• Adds focused tests under test/test_math.
• Updates coverage runs to set NUMBA_DISABLE_JIT=1 so coverage can trace jitted function bodies.
• Normal tests still run with JIT enabled.

Validation

• .venv/bin/pytest test/test_math -q -> 66 passed
• NUMBA_DISABLE_JIT=1 .venv/bin/pytest test/test_math --cov=src/spine/math --cov-report=term-missing -q -> 66 passed, 100% coverage
• bash check_coverage.sh test/test_math/ spine.math -> 66 passed, 100% coverage in Docker
• .venv/bin/python -m compileall -q src/spine/math test/test_math
• pre-commit hooks passed