[FIX] core: don't send readonly fields in o2m in SSF

The SSF would correctly filter out readonly fields when saving a
toplevel form, however it could not remove readonly values when saving
o2m pseudo-records to the parent form (as these would be expected to
remain available for reading upon the next edition and whatnot), so
these values would get sent in 0/1 commands.

Filter out these fields during the parent / toplevel save call.

Complexity notes:

* evaluating readonly modifiers requires the entire record, so
  unchanged fields still have to be written back to the parent form
  and be filtered out when *it* is set up for save, an alternative
  would be to store the `changed` and `readonly` flags alongside the
  record dict, and have the post-process only override the
  pre-computed readonly flag using force_save
* had to fix a test to match the new behaviour, the post-edition
  states turns out to be in line with the client's behaviour (or how
  it looks anyway)

Fixes #32019

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