ocda: heading cross reference linking

The segment a cross reference to a heading lands on, gixed upstream in
ocda, where the field is created, rather than inferred by the loader.

A heading above level 4 opens no html segment of its own, so a link to
it has to land on the level 4 heading that follows. The build loop
cannot know that when it reads the heading, and says so in a comment of
its own: "for html segname need following lv4 not yet known". It
back-fills tag_assoc when the level 4 heading arrives (lv0to3_tags), and
the answer then lives only in that map.

tags.segment_lv4_is now holds it, resolved in a pass over the finished
head and body sections: walking backwards, the last level 4 heading seen
is the next one for everything above it, which is the same answer the
back-fill gives. Emitted in the .ssp only where it differs from
.segment_html_is, which is every heading at level 4 or below, so it is
sparse: 317 lines over the 35 document reference set, none removed.
Carried in the database as a column of its own, and read back by both
readers.

docHasFromAbstraction reads it instead of working it out. With that, and
on top of the two defect fixes, a document loaded from an artefact and
one parsed from markup agree:

  key sets       identical, all 35 documents
  values         3 entries differ of some 30,000, all of them
                 _the_title, where the abstraction supplies an epub
                 segment the parser leaves unset
  link targets   all 1,651 agree, against 89 differing before any
                 of this work and 1 after the defect fixes alone

Format stays v1.1. That version is new in this same run of work and
nothing outside spine has read it, so this belongs in it rather than in
a bump of its own.

(assisted by Claude-Code)

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