site-i18n: a region in columns becomes one entry per column (#2467)

site-i18n: a region in columns becomes one entry per column (#2467)

* site-i18n: a region in columns becomes one entry per column

Baroque Art's credits page sets each role beside its people in Bootstrap
columns. Read as one Markdown text, the people came under the roles. A
region may now name a `rows` file: each column becomes an entry of its own,
its line breaks kept as backslash breaks, and the file lists the columns row
by row with their widths and the breaks legacy puts over a column's first
paragraph.

Part of #2404

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* site-i18n: a column is empty when no text is left of it, read by the converter

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>