Fix full-rewrite code fences with CRLF and literal language names

[emme1t]

parse_full_rewrite() retains the python fence label when an LLM response uses CRLF line endings, producing code that fails during evaluation. Language names are also interpolated as regular expressions: selecting c++ can extract an earlier c block on Python 3.12.

Escape the configured language name and accept CRLF after the opening fence. Regression tests cover LF/CRLF output, literal language selection, executable extracted Python, and the existing untagged/plain-text fallbacks.

Validation on Windows, Python 3.12:

• The new regressions fail on upstream 411fb59 in both reported cases; the updated utility suite passes all 14 tests.
• Full unittest discovery: 429 passed, with 4 failures/errors also reproduced on a clean upstream checkout (426 passed). These concern Windows temporary-file access, worker termination, and absolute-path expectations. Runs used UTF-8 mode and a dummy API key.
• Black, isort, and git diff --check pass for the changed files. Live-model integration tests were not run.

AI disclosure: Codex generated the patch and tests and performed automated review as part of an Astra Ultra-based automated PR project contributing to the open-source community.

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