NormalizeLineEndings(Mac) leaves a CRLF in the output and drops a line break when a CRLF is followed by an LF #137

What's wrong

StringExtensions.NormalizeLineEndings, Extensions/StringExtensions.cs:208:

LineEndingStyle.Mac => LineEndingRegexUnix.Replace(LineEndingRegexWindows.Replace(s, "\r"), "\r"),

The first pass turns every \r\n into \r. When a lone \n comes right after a \r\n, it then sits directly after the new \r. The second pass uses LineEndingRegexUnix ((?<!\r)\n), whose lookbehind rejects exactly that \n, so it is left alone. The two passes together build a new \r\n.

Repro

"a\r\n\nb".NormalizeLineEndings(LineEndingStyle.Mac)   // returns "a\r\nb", expected "a\r\rb"
"a\r\n\nb".NormalizeLineEndings(LineEndingStyle.Mac).DetermineLineEndings()   // Windows, not Mac

A local repro confirmed this: a\r\n\nb -> Mac: a\r\nb detect=Windows.

The other styles give correct results for the same input: Unix gives a\n\nb and Windows gives a\r\n\r\nb. Their pass order cannot create a new CRLF.

Why it matters

• The output still contains a Windows line ending, so the result is not Mac-normalized, and DetermineLineEndings() on it reports Windows.
• One of the two line breaks is lost, which changes the line count of the text.
• A CRLF followed by an LF is common in files with mixed line endings, which is exactly the input a normalizer is for.
• The existing NormalizeLineEndingsToMac test uses line1\nline2\r\nline3\r, which never puts an LF right after a CRLF, so it does not catch this.

Suggested fix

Do the conversion in one pass with a single alternation regex, so no intermediate string can create new sequences:

private static Regex AnyLineEndingRegex { get; } = new(@"\r\n|\r|\n", RegexOptions.Compiled);
// ...
LineEndingStyle.Mac => AnyLineEndingRegex.Replace(s, "\r"),

The same regex would also simplify the Unix, Windows, Mixed and None branches, which currently need two or three passes each.

Acceptance criteria

• "a\r\n\nb".NormalizeLineEndings(LineEndingStyle.Mac) == "a\r\rb"
• A test covers CRLF followed by LF, and CR followed by CRLF, for every LineEndingStyle.