Add XPath gap analysis and implementation proposal

Analyzed 448 unique XPath expressions across 3 Mendix test projects
to identify missing features. Key gaps: bare association paths (42%),
nested predicates (7%), not() existence checks (6%). Proposes a
dedicated XPath sub-grammar in 4 phases to reach ~98% coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako