The short form of a post or comment id: the first eight hex digits of
its UUID, e.g. 7ad26ccd.
Long lists of ids (a scheduling thread naming every eligible proposal)
read far better short, and people already write them that way. A prefix
is not an id: it may match nothing, or — rarely, at 32 bits over the
whole content table — more than one row. Only the server can say which,
so this type claims nothing beyond its shape, and every lookup must be
ready for an ambiguous answer. ContentIdPrefix::bounds gives the
inclusive UUID range it covers, for an index-friendly BETWEEN.
Parsing is lenient on case and surrounding whitespace; the canonical
form is lowercase. It is exactly eight digits and nothing else: a
truncated or mangled full UUID is an error, never quietly read as the
prefix it starts with.