Instead of going line-by-line, use split() with a giant regexp
to capture groups of contiguous lines. This offloads state
management to the regexp itself and makes it FAR easier to
keep track of <span> and </span> pairings.
Performance seems roughly on par after this change for the
meta@public-inbox archives. It seems a tiny bit faster for
git@vger with xt/perf-msgview.t, likely due to the longer
messages and larger contiguous groups of lines having the same
prefix (or no prefix at all) and drastically reduces the number
of subroutine calls and Perl ops executed.