Preserve entry type case

[claell]

Summary

• Preserve entry-type spelling from source while continuing to recognize block
  kinds case-insensitively.
• Prevent parse/write cycles from creating case-only changes.
• Lock in the related default preservation of citation-key and custom
  field-key spelling.
• Add an explicit NormalizeEntryTypes middleware for callers that prefer the
  former lowercase normalization.

Fixes #453. Related to the broader normalization discussion in #477.

Validation

• Focused splitter, public round-trip, and normalization regressions under
  warnings-as-errors: 18 passed.
• Focused identifier-case round-trip regressions under warnings-as-errors: 2
  passed.
• Complete upstream suite: 2,580 passed, 12 skipped, with the four existing
  deprecation warnings in tests/test_entrypoint.py.
• Complete pre-commit hook suite: passed.

Review note

This change was developed and validated in a concentrated session rather than
exercised over a long period in production. It includes an opt-in compatibility
path for lowercase normalization, but careful human review of the default
behavior and middleware API is requested.

AI assistance

This pull request was prepared with ChatGPT Codex using GPT-5.6 Sol with high
reasoning effort. Codex assisted with analysis, implementation, branch
isolation, and test execution. Automated validation is not a substitute for
maintainer review.

[MiWeiss]

Hi @claell — same note across all of your July 16 issues and PRs (#567–#597), apologies for the repetition.

Closing this. Passing CI isn't evidence the underlying claim is real — it just means generated tests match generated code. The batch's timing, structure, and forward-referenced numbers point to this being AI-generated rather than manually found and checked, and reviewing it properly would take more effort than a targeted pass over the parser myself.

If this fixes something you actually hit: open a small, human-verified PR against a reopened issue with a real repro, and I'll review it in good faith. Any nontrivial design or API choice needs to be discussed and agreed first, not decided inside a PR. Please disclose and verify AI-assisted work before submitting going forward.