fix: parse multiply shortcuts in universe data cards

[chenlichao]

Pull Request Checklist for MontePy

Description

Fixes #500.

This PR fixes parsing for MCNP universe data cards that use multiply shortcuts, for example:

U J 1 2M 5M J 3 7M 1M

Bug being fixed: MontePy did not correctly preserve expanded integer identifiers when a universe data card used shortcut tokens such as 2M, 5M, 7M, or 1M. Those values could fall back through the syntax tree as text-like values instead of the numeric values required by the universe parser.

Cause: NUM_MULTIPLY data-card tokens were not being converted through the same numeric-shortcut path used by other parsed numeric values. When ValueNode instances were later converted to integer identifiers, shortcut-expanded values were not consistently available.

Fix: The parser now treats NUM_MULTIPLY as a numeric shortcut token and preserves the expanded numeric shortcut result on the syntax node, so downstream integer conversion receives the correct value.

Syntax rule impact: This keeps MCNP multiply shortcut handling local to numeric data-card syntax: nM expands according to the existing shortcut semantics, and the expanded values are used when constructing universe identifiers. The regression test covers the concrete issue input and verifies the resulting universe sequence.

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General Checklist

• I have performed a self-review of my own code.
• The code follows the standards outlined in the development documentation.
• I have formatted my code with black version 25 or 26.
• I have added tests that prove my fix is effective or that my feature works (if applicable).

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LLM Disclosure

1. Are you?

   • A human user
   • A large language model (LLM), including ones acting on behalf of a human

2. Were any large language models (LLM or "AI") used in to generate any of this code?

• Yes
  • Model(s) used: GPT-5.5 (OpenAI Codex), via Hermes Agent
• No

Documentation Checklist

• I have documented all added classes and methods. Added changelog entry in doc/source/changelog.rst.
• For infrastructure updates, I have updated the developer's guide.
• For significant new features, I have added a section to the getting started guide.

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First-Time Contributor Checklist

• If this is your first contribution, add yourself to pyproject.toml if you wish to do so.

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Additional Notes for Reviewers

Ensure that:

• This PR fully addresses and resolves the referenced issue(s).
• The submitted code is consistent with the merge checklist outlined here.
• The PR covers all relevant aspects according to the development guidelines.
• 100% coverage of the patch is achieved, or justification for a variance is given.

Test Plan

• .venv/bin/python -m pytest tests/test_universe.py -q — 40 passed
• cd doc && make html SPHINXOPTS='-W --keep-going -E' — build succeeded

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📚 Documentation preview 📚: https://montepy--975.org.readthedocs.build/en/975/

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📚 Documentation preview 📚: https://montepy--975.org.readthedocs.build/en/975/

[tjlaboss]

Please use the pull request template.

AI contributions are allowed, but must be disclosed.

[MicahGale]

@chenlichao if the PR template is not used this PR will be closed in one week.

[MicahGale]

@chenlichao please expand your description to include things like:

1. What the bug being fixed is.
2. What was causing the bug
3. How this bug was fixed.
4. How exactly the new syntax rules fix this bug.

For fixing the test failures:

1. changelog-test please update doc/source/changelog.rst accordingly.
2. doc-build this should now be fixed in develop just merge or rebase that in.

[chenlichao]

Thanks for the guidance. I rebased this onto current develop, added the changelog entry, and expanded the PR description to cover the bug, root cause, fix, syntax behavior, and LLM disclosure. I also verified tests/test_universe.py and the strict docs build locally.