The ImGui Cmd+V/Ctrl+V paste handler split clipboard text only on '\n',
leaving '\r' inside each segment. Pasting text with CRLF (Windows) or bare
CR (classic-Mac / some macOS apps) line endings enqueued an InsertText arg
containing '\r', which the InsertText command rejects with "InsertText arg
must not contain newlines".
Extract the paste-to-commands logic into a pure, SDL-free helper
(PasteSplit.h) that treats '\n', '\r\n', and bare '\r' each as one line
break, and add unit tests covering all line-ending forms plus blank-line
and trailing-newline preservation.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
