Prepare ruby-dag v1.5.0 with Mutant gate

[duncanita]

Summary

• Add Mutant/Minitest mutation testing config, rake tasks, and the spec bridge Mutant needs for this repo layout.
• Harden graph, result, execution-context, and effect-await coverage while accepting equivalent simplifications surfaced by Mutant.
• Prepare ruby-dag v1.5.0 with version bump, changelog, roadmap, and a release gate test.

Validation

• bundle exec rake
• bundle exec rake mutant:test
• MUTANT_JOBS=4 bundle exec rake mutant:run (3285/3285 killed, Alive: 0, Coverage: 100.00%)
• git diff --check

Notes

• Mutant prints the non-fatal parser/current Ruby 3.3 parser warning while running on Ruby 4.0.2.
• Untracked local review artifacts were intentionally left out of the branch.