[Non-blocking] Minor code quality issues: unused imports and type annotations

[raphaelreinauer]

Per the JOSS review checklist:

Functionality: Have the functional claims of the software been confirmed?

All functionality works correctly (101 tests pass), but static analysis show some issue:

1. Unused imports:

> ruff check src/ect/

src/ect/embed_complex.py:12-14: F401 - unused imports: point_in_polygon, validate_face_embedding, validate_edge_embedding
src/ect/sect.py:5: F401 - unused import: Union

2. Type annotation issues:

> mypy src/ect/ --ignore-missing-imports

• 8 implicit Optional violations (PEP 484) in validation/rules.py and validation/base.py
• Type mismatches in directions.py (lines 117, 126, 131, 147, 157, 164)

These issues are non-blocking but would be great to address.

Part of this JOSS review.