test: add offline GitLab corpus verification

[ebihara99999]

Purpose

Refs #41. This adds the first CI layer from the corpus verification plan: vendored corpus files, a per-file runner, crash checks, and metric-agnostic invariants.

RuboCop differential checks and golden snapshots are intentionally left for follow-up PRs.

Changes

• Vendor the selected GitLab CE Ruby corpus from revision 85ed8239cfc6fe3cf5d5bde5975fe2f6d687ff6d under spec/fixtures/corpus/gitlab/.
• Include the GitLab license notice and a corpus README with the source revision, file list, placement, and ee/, jh/, doc/ exclusion rule.
• Add an RSpec corpus runner that measures each vendored file with explicit CodeKeeper metric and thread settings.
• Add CI-covered checks for crash-free per-file measurement, report shape, summary recomputation, ABC/cyclomatic count alignment, line ranges, sequential/parallel determinism, and the current behavior that metrics with no measurements are absent from summary.
• Exclude spec/fixtures/**/*_spec.rb from RSpec discovery so vendored GitLab spec files remain corpus input data.

Impact

This is test-only. The corpus is under spec/fixtures/, remains outside the packaged gem, and is already outside RuboCop's scan by the repository configuration.

Verification

• bundle exec rspec - 75 examples, 0 failures
• bundle exec rubocop - 34 files inspected, no offenses
• git diff --cached --check - no whitespace errors before commit

Scope Out

• RuboCop differential and accepted-differences baseline
• Golden JSON snapshots and regeneration task