Apply terminal detection for the refinement entry point

[OskarEichler]

Summary

Route the documented standalone require 'rainbow/refinement' entry point through Rainbow's normal initialization so it honors TTY detection, TERM=dumb, and CLICOLOR_FORCE.

Reproduction

With output redirected, require 'rainbow' disables formatting, but require 'rainbow/refinement'; using Rainbow currently leaves formatting enabled because it loads global directly and skips lib/rainbow.rb initialization.

Verification

• Existing suite: 220 examples, 0 failures on Ruby 3.2.11 and 4.0.6.
• RuboCop: 15 files, 0 offenses on both Rubies.
• External isolated-process model: 66 checks per Ruby covering STDOUT/STDERR TTY combinations, normal/dumb terminals, force values, both require orders, real redirected output, and explicit post-require enable/disable. Current master/release fail 15 cases per Ruby; this branch passes all.
• Rebuilt gem: 14 expected files and successful packaged loads on both Rubies.
• No test files changed.

Compatibility

No API break. The refinement entry point now follows the same documented automatic enable/disable policy as require 'rainbow'. Explicit Rainbow.enabled= overrides remain effective.