Make definition nesting usable for constant resolution

[soutaro]

Summary

• Add recursive constant resolution for API-created name chains.
• Make Ruby Graph#resolve_constant use recursive resolution so rooted nesting frames resolve correctly.
• Add source_name for class, module, and constant definitions so callers can preserve source-level constant paths like ::Bar.

Tests

• cargo test -p rubydex recursive_constant_resolution
• cargo test -p rubydex-sys
• shadowenv exec -- bundle exec rake compile
• shadowenv exec -- bundle exec ruby -Itest test/graph_test.rb
• shadowenv exec -- bundle exec ruby -Itest test/definition_test.rb -n /source_name/