Modernize CI/test runner: Appraisal-based matrix mirroring solid-process

[serradura]

Summary

• Replaces the per-Rails subdirectory gemfiles/rails_X.Y/Gemfile layout with an Appraisals file + auto-generated gemfiles/rails_X_Y.gemfile, matching the solid-process gem layout.
• Expands the CI matrix to Ruby 2.7, 3.0, 3.1, 3.2, 3.3, 3.4, 4.0, head × Rails 6.0, 6.1, 7.0, 7.1, 7.2, 8.0, 8.1, edge (gated per Ruby/Rails compatibility).
• Preserves the ENABLE_TRANSITIONS=true/false matrix dimension.
• Adds bin/setup, bin/matrix, and a rake matrix task that loops every applicable appraisal × both transitions modes.
• Drops Rails 5.2 from the matrix (no longer in solid-process's range; u-case runtime has no hard Rails dep).
• Drops the CodeClimate coverage upload step (was hardcoded; reintroduce per preferred provider — Qlty/CodeClimate — as a follow-up).

Test infra fixes discovered while wiring this up

• test/test_helper.rb required pry-byebug unconditionally, which blocked any env without the dev gem. Removed.
• test/micro/case/result_test.rb#test_inspect hard-coded the pre-Ruby-3.4 hash inspect format ({:k=>v}). Now derives the expected string from Hash#inspect so it works across Ruby versions.

Open follow-ups (not in this PR)

• required_ruby_version in the gemspec still says >= 2.2.0 — bumping to >= 2.7 would match the new CI floor.
• Add a coverage uploader (Qlty/CodeClimate) when a token is available.
• appraisal install only generated rails_8_1.gemfile and rails_edge.gemfile locally (Ruby 4.0 is the only Ruby on the dev box, and the Appraisals file gates older Rails to older Rubies, mirroring solid-process). CI regenerates the rest per Ruby version via bundle exec appraisal install.

Test plan

• CI passes (or surfaces real Ruby/Rails-combo regressions that can be addressed in follow-up commits)
• bin/setup then bin/matrix runs locally end-to-end on at least one Ruby version
• Locally verified on Ruby 4.0.1: bundle exec rake matrix runs rails-8-1 + rails-edge × transitions true/false → 133 tests, 0 failures (4 jobs)

🤖 Generated with Claude Code