refactor: replace hardcoded string literals with tuple constant references

refactor: replace hardcoded string literals with tuple constant references

Replace 8 hardcoded string literal comparisons with proper tuple constant
references using CONSTANT[index] pattern:

- _DENSITY_AGGREGATIONS: 'quantile' and 'power_mean' comparisons (2 fixes)
- _DISTANCE_METHODS: 'topsis' and 'compromise_programming' comparisons (6 fixes)

This improves type safety, maintainability, and ensures consistency with
the codebase's established constant usage patterns.