Preserve CRLF blank lines during float-to-top preprocessing - #2683

Fixes #2681.

Count each trailing CRLF as one complete newline during float-to-top preprocessing, and restore blank lines using the parsed/configured line separator. Previously the scan stopped at \r, so the normal sorting pass restored a lost blank line and reported a change even when the final output was identical to the input. This also made --check fail without a diff.

Added 17 cases covering split/off/on boundaries, inline split comments, spacing counts, configured endings, genuinely unsorted input and a byte-preserving CLI check. Eight fail on unmodified main. This branch is based directly on main and does not depend on #2682.

Validation on Windows / Python 3.13.15:

• 669 unit tests passed, 5 existing skips; 3 failures reproduced independently on unmodified main (test_gitignore, test_cli_src_path_glob_pattern, test_find_config).
• 3 local integration/property tests passed; coverage is 99.26%, with core.py at 100% including branches.
• Locked-version Ruff/format, flake8, isort on tracked Python files excluding the repository-declared CRLF fixture, Linux-targeted mypy, dependency consistency, clean-export wheel/sdist and Twine passed.
• Bandit still reports the unchanged B101 assertion in parse.py. Compiled wheels and downstream repository-cloning integration tests were not run.
• A simulated merge with independent Fix split directives on complete multiline imports #2682 is conflict-free; all 34 new cases pass together.

Assisted by OpenAI Codex.