treat a carriage return as a line break when emitting

[soma0212]

The scanner takes a bare CR as a line break (Exp::Break, after #986 and #1309), but the emitter helpers still only look for LF:

• WriteComment re-prefixes # on LF only, so Comment("hi\rinjected: pwned") on a one-key map reparses as two keys, the comment tail landing as document structure
• IsValidSingleQuotedScalar rejects LF but not CR, so 'a\rb' goes out raw and folds back to a b
• IsValidLiteralScalar checks for neither, so a CR in a block scalar comes back as LF
• WriteSingleQuotedString keeps its own bail-out in step with its validator

Plain scalars already fall back to double quotes for this (#607); the two remaining explicit formats now do the same, while the comment writer handles the break in place since it owns the # continuation. A CRLF pair collapses to one break instead of two.