Preserve source maps while generating map assets

[OskarEichler]

Summary

• copy an asset's source map before assigning the generated map asset's file
• preserve caller ownership and support frozen input maps
• document the behavior in README and CHANGELOG

Verification

• focused external ownership/frozen-input 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.