The variant handler now supports prefix matching for directional variants.
This allows 'pa hi' to match 'parry high', 'pa lo' to match 'parry low', etc.
Implementation:
- First tries exact match on variant key
- Falls back to prefix matching if no exact match
- Returns unique match if exactly one variant starts with the prefix
- Shows disambiguation message if multiple variants match
- Shows error with valid options if no variants match
Tests cover exact match, prefix match, ambiguous prefix, no match,
single-char prefix, case-insensitivity, and preservation of target args.
