Support nested maps when concatenating source maps

[OskarEichler]

Summary

• preserve both source-map inputs while concatenating
• recursively concatenate nested index maps, retaining section offsets and rebasing leaf sources
• consume private line-count metadata only in the copied result
• document the behavior in README and CHANGELOG

This consolidates and supersedes the narrower input-ownership change in #845.

Verification

• 10 focused nested-concatenation assertions 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.