test: add RuboCop metric differential baseline

[ebihara99999]

Purpose

Refs #41. This is stacked on #43 and implements the second PR-plan slice: RuboCop differential checks with an accepted-differences baseline.

Golden snapshots and regeneration tasks remain out of scope for a follow-up PR.

Changes

• Run the vendored corpus through RuboCop metric cops via RuboCop::Cop::Team with forced in-memory configuration instead of repository .rubocop.yml.
• Pin TargetRubyVersion explicitly to the running Ruby major/minor used by CodeKeeper parsing.
• Use test-only wrapper cops to capture raw metric values before RuboCop rounds offense messages.
• Compare CodeKeeper and RuboCop measurements bidirectionally and uniquely by metric, path, and start line.
• Fail on value mismatches, duplicate match keys, new unaccepted differences, and stale baseline entries.
• Seed the baseline with the known singleton-class behavior differences described in Add corpus-based verification for the metric report #41.
• Include Metrics/ModuleLength in the RuboCop oracle with the same CodeLength settings because CodeKeeper's class_length metric covers module scopes as well.

Impact

This is test-only. It adds CI coverage over the vendored corpus introduced by #43 and does not change runtime library behavior.

Verification

• bundle exec rspec - 77 examples, 0 failures
• bundle exec rubocop - 39 files inspected, no offenses
• git diff --check - no whitespace errors

Scope Out

• Golden JSON snapshots
• Snapshot regeneration task