[codex] Extract Rails compiler artifact path helpers

[fullofcaffeine]

What Changed

• Extracted Rails artifact path normalization and fail-closed path validation into reflaxe.ruby.rails.RailsArtifactPaths.
• Left small delegating wrappers in RubyCompiler.hx so existing call sites stay stable while the compiler is split incrementally.
• Added docs/ruby-compiler-rails-module-extraction.md to map RailsHx compiler surfaces, target module boundaries, extraction order, and coverage expectations.
• Added AGENTS guidance that new RailsHx compiler behavior should prefer focused modules over adding more bulk to RubyCompiler.hx.
• Closed bead haxe.ruby-67c with code/docs/bead state together.

Why

RubyCompiler.hx has become the orchestration root plus the home for many RailsHx surfaces. This starts the refactor with a low-risk, behavior-preserving extraction that is deterministic and snapshot-covered, while documenting the larger module plan.

Validation

• npm run format:haxe
• npm run format:haxe:check
• npm run test:action-mailer
• npm run test:template-test-generator
• npm run test:rails-integration
• npm run test:components
• npm run test:snapshots
• git diff --check
• GitHub Actions green on pushed branch: https://github.com/fullofcaffeine/reflaxe.ruby/actions/runs/28420261144