Fix source spans for unclosed reference titles

[sean0407]

Fixes #460.

When a link reference definition has an unclosed title that starts on a later line, the parser currently keeps the partial title and assigns that line's source span to the definition. The same line remains paragraph content, but its paragraph node has no source span.

Record where title spans begin. If the paragraph ends while the parser is still reading a title, discard the partial title and leave those later-line spans with the paragraph. The regression test checks the definition title and both nodes' source spans.

Verification:

• git diff --check passed.
• Project patch policy accepted: 2 files, 39 changed lines, no binary changes.