Resolve sources from nested index maps

[OskarEichler]

Summary

• recursively enumerate leaf sources from nested index source maps
• resolve each leaf through the existing environment path handling
• document nested source resolution in README and CHANGELOG

Verification

• focused external nested-map resolution model on Ruby 4.0.6 and 3.2.11
• unchanged source-map suites: 40 runs, 768 assertions on each Ruby
• full suite: 913 runs, 4,109 assertions, zero failures/errors on each Ruby
• RuboCop and package-surface checks pass

No test files are changed.