Report unsupported parenthesis-delimited blocks

[claell]

Summary

• Detect standard parenthesis-delimited entries, strings, preambles, and
  comments instead of classifying them as implicit comments.
• Retain their exact source in explicit ParsingFailedBlock objects while full
  parenthesis parsing remains unsupported.
• Recover later supported blocks on the same or following line, including after
  an unclosed parenthesis block.

Fixes #533 by taking the issue's fail-closed interim option. This does not claim
to implement full parenthesis-delimited parsing.

Validation

• Focused splitter detection and recovery suite under warnings-as-errors: 32
  passed.
• Complete upstream suite: 2,585 passed, 12 skipped, with the four existing
  deprecation warnings in tests/test_entrypoint.py.
• git diff --check: passed.
• The project pre-commit executable was not available in the isolated local
  environment; GitHub CI should independently run the configured hooks.

Review note

This change was developed and validated in a concentrated session rather than
exercised over a long period in production. The recovery scanner handles nested
parentheses, braces, quotes, and subsequent blocks, but careful human review of
malformed-input edge cases is particularly important.

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.