Preserve line endings when applying patches

[charliemarsh-oai]

Summary

• preserve each untouched source line's existing LF or CRLF terminator when applying a patch
• use the first detected source terminator for inserted and replaced lines, defaulting to LF when the file has none
• retain parsed context-line identity so repeated or fuzzily matched lines preserve the correct source occurrence
• rebuild updated files from sorted replacement ranges in one forward pass

Why

apply_patch currently splits source contents on \n, which leaves \r attached to existing CRLF lines, while replacement lines parsed from the patch contain no \r. Joining the result with \n therefore preserves CRLF on untouched lines but emits LF on changed lines, producing mixed-ending files.

The previous fix in #7515 intentionally applied only to Windows. It stripped terminators with str::lines, classified the entire file as CRLF only when every newline was CRLF, and then regenerated the whole file with one separator. That fixed uniform CRLF files, but it coupled behavior to the host OS and normalized mixed-ending files instead of preserving untouched source. The change was reverted wholesale by #7903, leaving the original split/join behavior in place.

Design

This follows the line-ending design we use in Ruff. Ruff's universal newline model recognizes LF, CRLF, and CR and exposes the actual terminator for each source line. Ruff's LineEnding::Auto uses the first detected newline as the preferred style and defaults to LF.

SourceFile applies the same preferred-style rule for new text while remaining more conservative than a formatter: untouched lines retain their exact original terminators, including in mixed-ending files. The patch parser also records which old/new line pairs were context rather than trying to infer equality after parsing. Replacement ranges can then exclude those exact context lines, even when adjacent changed lines have identical text or matching required whitespace normalization.

Closes #4003.