Make ruby-coverage optional until capture starts

[samuel-williams-shopify]

TL;DR

Make ruby-coverage optional until coverage capture is actually started, so consumers can load covered/sus without installing or building the native ruby-coverage extension.

Context

This came up while fixing socketry/io-event CI. io-event loads covered/sus from its Sus configuration, but normal test jobs do not set COVERAGE. Even though Covered::Sus already avoids loading Covered::Config unless COVERAGE is set, covered still declared ruby-coverage as a runtime dependency and Covered::Policy eagerly required covered/capture, which required ruby/coverage.

That means applications can pay the native dependency install/build cost even when they are not recording coverage. On platforms where ruby-coverage is not usable, that fails before tests get to run.

Changes

• Move ruby-coverage from a runtime dependency to a development dependency.
• Defer covered/capture loading until Covered::Policy#capture is used.
• Keep an explicit ruby/coverage require in Covered::Capture, but convert missing dependency failures into a targeted LoadError explaining what gem to add when recording coverage.
• Add a Covered::Sus regression test that runs in a child Ruby process with a trap ruby/coverage.rb earlier in $LOAD_PATH; requiring covered/sus with COVERAGE unset must not trigger that trap.

Tophatting

PATH=/Users/samuel/.local/state/tec/profiles/base/current/global/bin:$PATH /Users/samuel/.local/state/tec/profiles/base/current/global/bin/bundle exec bake test
# 71 passed out of 71 total

/Users/samuel/.local/state/tec/profiles/base/current/global/bin/bundle exec sus test/covered/sus.rb
# 4 passed out of 4 total

[samuel-williams-shopify]

Closing this draft. The better fix is to keep Covered::Sus as-is, since it already defers configuration loading, and handle the optional coverage dependency in the consumer workflow/bundle instead.