refactor: improve string literal replacements and use specific tuple constants

refactor: improve string literal replacements and use specific tuple constants

- Replace hardcoded method names in error messages with tuple constants
  (lines 2003, 2167: use _DISTANCE_METHODS[0] and [1] instead of literal strings)
- Use _CLUSTER_METHODS instead of _SELECTION_METHODS indices for better
  code maintainability (e.g., _CLUSTER_METHODS[0] vs _SELECTION_METHODS[2])
- Fix trial_selection_method comparison order to match tuple constant order
  (compromise_programming [0] before topsis [1])
- Remove redundant power_mean None check (already validated by _validate_power_mean)
- Add clarifying comments to tuple constant usages (minkowski, rank_extrema)