Improve test and documentation coverage

[samuel-williams-shopify]

Summary

• Add focused coverage for monitor trace-point dispatch and simple guard violations.
• Split composite attr_reader declarations into individually documented attrs.
• Keep coverage validation dependent on the combined Ruby matrix so the Ruby 3.2 ObjectSpace::WeakMap fallback is covered by the Ruby 3.2 shard.

Verification

• bundle exec bake agent:context:install
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• bundle exec bake test
• COVERAGE=PartialSummary bundle exec bake test

Note: the local Ruby-current coverage run reports only the Ruby 3.2 WeakMap fallback as uncovered. CI combines Ruby 3.2 and current coverage artifacts to validate both branches.