0.13.20 — part number

Series read in order (`ordering: "sequence"`) list their entries from the first one. The order was right, but the tiles did not say which part was which. With a serialised series, a reader coming back a week later had to count tiles from the top to know where they stopped.

Since 0.13.20 an entry can have a `part` field — the part number. The tile puts it before the title: “1. Title”, “2. Title”. This happens wherever there is a tile, which means the series list and the stream, where the number helps most, because entries from other series stand next to it.

The number is given explicitly rather than derived from `order`. The reason is practical: a serialised series often opens with an introduction that comes first in the order but is not a part. Numbering derived from the order would shift everything by one. An entry without `part` looks exactly as before.

The title itself stays unchanged. The entry page, the RSS feed and social preview cards carry the title without a number — the number tells you the place in the list; it is not part of the text's name.