Suppress WA515 for conditional duplicate labels

[nonprofittechy]

Fixes #62

Summary:

• Only emit WA515 when at least two same-label fields are always visible.
• Treat show if/hide if/js show if/js hide if as visibility logic for duplicate-label suppression.
• Add regression tests for always-visible duplicates, contextual Other fields, and hidden duplicates.

Tests:

• python -m unittest tests.test_yaml_structure