parallelise: show_abstraction & ocda_db as the rest

Fix issue with consistency (flags run serial & parallel
inconsistenly).

Both write one file per document per language and share no
handle, so they belong on the list with html, epub, text and
sqlite_discrete.

The guard above the list still takes out --pod, --pod2,
--source and the shared sqlite db actions before it is reached, so
those stay serial as they were; checked.

However measurement tests show parallel turn out to result in a
processing slowdown, on the (35) sample markup documents, run on
multiple passes: run on 16 cores a slow down of about 20% for
eleven times the cpu!
  --text                           4.83-5.10 s wall   48-52 s user
  --text --serial                  3.94-4.05 s wall   4.2-4.4 s user

will make serial run the default.

(assisted by Claude-Code)

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