Replace virtus with an internal attributes module

[pnomolos]

Why

virtus is unmaintained and pulls in a chain of abandoned gems
(axiom-types, coercible, descendants_tracker, ice_nine) plus the
deprecated thread_safe (superseded by concurrent-ruby). RubyCritic only
used a tiny slice of virtus -- an attribute DSL on two classes -- so
carrying that whole dependency tree wasn't justified.

A custom module was chosen over dry-rb deliberately: dry-struct would just
trade one transitive tree for another (dry-core, dry-types, dry-logic,
zeitwerk...), which runs counter to the goal and to the codebase's recent
direction of shedding/leaning out dependencies (e.g. preferring prism over
the parser gem, dropping cucumber).

What changed

• New internal RubyCritic::Attributes module reproducing only the used
  virtus subset: an attribute macro, type coercion, per-instance duped
  defaults, and a hash-accepting initializer.
• Smell and AnalysedModule now include RubyCritic::Attributes instead
  of Virtus.model.
• virtus runtime dependency removed; ostruct moved to a development
  dependency (only used in tests).
• .reek.yml and CHANGELOG.md updated accordingly.

Verification

• Full rake suite is green: tests, reek (0 warnings), and rubocop (no
  offenses).
• The full virtus dependency chain is confirmed gone from the bundle.
• Behavior verified faithful to virtus: mutable defaults duped per-instance,
  Float::INFINITY/Symbol defaults preserved, coercion applied on writers,
  and method overrides intact.

Checklist

• Add an entry to the changelog
• Squash all commits into a single one
• Describe your PR, link issues, etc.

🤖 Generated with Claude Code

[pnomolos]

Superseded by the upstream PR against whitesmith/rubycritic (whitesmith#574).