Add mutant gem for open-source mutation testing

[stefan-burke]

Summary

Installs the mutant mutation-testing engine in open-source mode and wires it up for the RSpec suite, then runs it against several core models to measure how thoroughly their tests actually pin down behaviour.

Mutation testing mutates the source (e.g. > → >=, deleting an argument, flipping a boolean) and re-runs the covering tests. A mutation that survives means no test noticed the change — a genuine gap even when line coverage is 100%.

Changes

• Gemfile — add mutant and mutant-rspec (require: false) to the development, :test group.
• Gemfile.lock — resolved dependencies (mutant 0.13.1, unparser, etc.).
• config/mutant.yml — project config: usage: opensource, rspec integration, requires ./config/environment, sets RAILS_ENV=test, and lists example subjects.
• bin/mutant — bundler binstub.

Usage

bundle exec mutant run -- 'InspectorCompany'
bundle exec mutant run -- 'TextReplacement#tree_structure'
bundle exec mutant environment subject list   # list mutable subjects

Mutation results

Run against three core models and their specs:

Subject	Mutations	Killed	Alive	Coverage
TextReplacement	196	191	5	97.45%
InspectorCompany	250	224	26	89.60%
Event	121	81	40	66.94%

Higher is better — surviving ("alive") mutations are behaviours no test asserts on. TextReplacement is very well covered; Event has the most gaps (notably Event.log, Event#resource_object, and Event#description). These highlight where the existing tests could be strengthened.

Credential was also configured but has no mutable subjects — it declares only validation macros, no method bodies for mutant to mutate.

Notes

• Open-source mode (usage: opensource) requires no licence key for public repositories.
• The gems are scoped to development/test only.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CLseCgvqE2jdan8K57ENJn

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