feat(prosediff): recover the source line each word edit sits on (spec-by6.3.5)

The review UI is moving to a line-numbered unified diff, which needs to know
which line a word-level change happened on. The differ does not keep that.
Tokenize drops whitespace — "\n" and " " both collapse to Token.Space — and
that is precisely what makes a rewrapped paragraph produce a byte-identical
token stream and therefore no diff at all. The property is load-bearing, so the
line is recovered here rather than retained there.

It is recoverable because the script is ordered: the equal and deleted runs
reproduce the old block's tokens in sequence, and the equal and inserted ones
the new block's. Walking each side in step with that side's re-tokenized lines
says which line every token belongs to, and a run crossing a line break is cut
at the boundary.

The script supplies only the operation per token; text and spacing come from
re-tokenizing the source line. Taking text from the spans instead drops
separators — Span.Space is false on an insertion that directly replaces a
deletion, because in a combined rendering the deletion before it carried the
space, and split onto one side that deletion is gone. Caught by a test:
"delta CHANGED zeta" rendered as "deltaCHANGED zeta".

Reports ok=false rather than guessing when a block's Lines and Text disagree
about token count. A caller that cannot split falls back to rendering the block
as one old/new pair labelled by line range: a wrong line number is worse than
an honest range, because it invites a comment onto text that was never there.

spec-by6.3.5