prefix every line of a multiline comment in Item.comment and add_line

[netliomax25-code]

Summary

1. Item.comment() and Array.add_line(comment=...) prepend a single # to the whole comment string, so the second line onward of a comment with an embedded newline renders without a #.
2. That output no longer re-parses: the bare continuation line is read as a stray key/token, so tomlkit raises a parse error and tomllib rejects it too.
3. Prefix every line of a multiline comment with # #511 fixed this for the comment() factory but left these two public-API siblings. Both now go through one helper that prefixes every line, matching that fix. Single-line comments are unchanged.

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