Metrics: custom metrics

[LukasNiessen]

custom metric(name, description, fn) where the function receives the full class information, plus should satisfy(fn) taking (value, classInfo).

This is the escape hatch that stops the metrics module needing to be exhaustive.

[TristanKruse]

Implemented and pushed in f17fd69, with integration in 34062aa and RAG fixture coverage in TristanKruse/ArchUnitRuby-TestRepo-RAG@c5c99b8. Adds custom_metric(name, description, fn), custom measurements, and should_satisfy(value, class_info), including structured violations and the universal empty-selection guard. The other five generic threshold predicates remain scoped to #36. Verified with 404 examples, 99.20% line and 92.63% branch coverage, packaged-gem smoke tests, the RAG fixture, and final hosted CI: https://github.com/LukasNiessen/ArchUnitRuby/actions/runs/31408138410