fix: replace O(n²) path dedup with O(1) hash set lookup

[dangell7]

High Level Overview of Change

Context of Change

addUniquePath was using linear scan to check for duplicate paths,
resulting in ~500k comparisons at PATHFINDER_MAX_COMPLETE_PATHS=1000.

• Expose STPathElement::hash() accessor (existing precomputed field)
• Add STPathHash functor combining element hashes via boost hash_combine pattern
• Maintain a parallel unordered_set index alongside completePaths_ for O(1) dedup

The original code had a TODO acknowledging the issue:

// TODO(tom): building an STPathSet this way is quadratic in the size
// of the STPathSet!

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)