fix string escaping in plain format

[wtn]

Fixes data corruption when round-tripping strings containing backslashes or newlines in plain format.

Changes

• Fix backslash escaping in CFString#quoted (output \\ instead of \)
• Fix newline escaping in CFString#quoted (output \n instead of literal newline)
• Fix regex in CFString#to_plain to use \A\z instead of ^$ for proper string anchoring
• Add roundtrip tests for strings with backslashes and newlines

Background

Strings with special characters were corrupted during plain format serialization:

• Backslashes were not properly escaped, causing "foo\bar" to become "foobar"
• Newlines were output as literal characters instead of \n, breaking the parser
• The regex check used line anchors instead of string anchors, causing multiline strings to bypass quoting

[ckruse]

@wtn you did a lot of work. Thank you very much!