Review/adjust MARC importer for LC "Modern MARC"

### Feature Request

## Problem / Opportunity

The Library of Congress and the greater bibliographic cataloging community change their cataloging practices in an ongoing fashion. The latest iteration is documented here: https://www.loc.gov/marc/ModernMARC-March2026.pdf

## Proposal

The new cataloging guidelines should be reviewed with an eye towards changes which need to be made in the MARC importer to either take advantage of new features/information or changes to previous practices which are relied on by the importer.

@hornc

[ModernMARC-March2026.pdf](https://github.com/user-attachments/files/26391221/ModernMARC-March2026.pdf)

### Breakdown

Implementation Details (for maintainers)

#### Related files

Refer to [this map of common Endpoints](https://docs.openlibrary.org/developers/backend/endpoints.html):
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#### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
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#### Stakeholders

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#### Instructions for Contributors

* **Before** [creating a new branch](https://docs.openlibrary.org/developers/tools/git.html#making-changes-and-creating-a-pull-request) or pushing up changes to a PR, please first [run these commands](https://docs.openlibrary.org/developers/tools/git.html#working-on-your-branch) to ensure your repository is up to date, as the pre-commit bot may add commits to your PRs upstream.

Start by reading the linked ModernMARC-March2026.pdf and then locate the repository's MARC importer entry point. Compare the documented cataloging changes with the importer's current assumptions, identify affected behavior, and define tests or fixtures that demonstrate the required adjustments before making changes.

Written by the indexing model from the issue text.