fix(rubric): represent DIF applicability explicitly in item-bank calibration

[seonghobae]

Supersedes draft #787 (unique item-bank DIF slice only). Local item-bank tests green. Merge when product gates pass.

Summary by CodeRabbit

• New Features
  • Calibration now supports explicit evidence that differential item functioning (DIF) does not apply.
  • Calibration requires either DIF evidence or approved non-applicability evidence, alongside existing quality checks.
  • Conflicting DIF and non-applicability evidence is rejected.
• Documentation
  • Added changelog documentation covering the updated calibration evidence requirements.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: e0212e8d-3b47-4076-9203-c722e14fa7ac

📥 Commits

Reviewing files that changed from the base of the PR and between 5231438 and 863d658.

📒 Files selected for processing (3)

• docs/changelog.d/787-item-bank-dif-applicability.md
• python/fast_mlsirm/rubric/item_bank.py
• tests/test_item_bank_dif_applicability.py

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📝 Walkthrough

Walkthrough

Calibration transitions now accept either measured DIF or explicit DIF non-applicability evidence. They reject both evidence kinds together and retain existing lifecycle requirements. Tests cover the new evidence kind and transition outcomes.

Changes

Item-bank calibration evidence

Layer / File(s)	Summary
Calibration DIF applicability rules python/fast_mlsirm/rubric/item_bank.py, tests/test_item_bank_dif_applicability.py, docs/changelog.d/787-item-bank-dif-applicability.md	Adds DIF_NOT_APPLICABLE, requires an explicit DIF applicability decision during calibration, rejects conflicting evidence, and tests measured-DIF and non-applicability paths.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: explicit DIF applicability representation in item-bank calibration.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/item-bank-dif-applicability-green

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