Summary

• Recognize ordinary BibTeX whitespace, including line endings, before outer block delimiters.
• Preserve punctuation permitted in entry-type identifiers instead of treating the candidate as an implicit comment.
• Require the structural matrix to produce either a parsed block or an explicit failed block; no candidate may disappear silently.

Closes #583.

Dependency and commit structure

This draft currently includes the exact prerequisite behavior from:

• PR Report unsupported parenthesis-delimited blocks #571, which retains unsupported parenthesis-delimited blocks as explicit failures; and
• PR Require exact special command types #574, which requires exact matching for @comment, @preamble, and @string.

Those behaviors remain separate as the first two commits. The issue #583 implementation and its exhaustive visibility matrix are the final two commits. The branch is executable against current main; once the prerequisites merge, it should be rebased so their already-reviewed diffs disappear from this PR.

Validation

• pytest -p no:cacheprovider -W error tests/splitter_tests/test_splitter_block_start_detection.py: 75 passed.
• Complete upstream suite: 2,628 passed, 12 skipped, with the four existing deprecation warnings in tests/test_entrypoint.py.
• Complete pre-commit hook suite: passed.

Review note

This changes the splitter's block-start grammar, so the draft should receive careful maintainer review despite the broad structural matrix. In particular, review should confirm the reserved-character set used to terminate entry-type identifiers and the decision to keep parenthesis syntax explicit but unsupported for now.

AI assistance

This pull request was prepared with ChatGPT Codex using GPT-5.6 Sol with high reasoning effort. Codex assisted with parser analysis, dependency separation, conflict resolution, test design, and validation. Automated validation is not a substitute for maintainer review.