When two figures share the same figure_id (e.g. supplementary
"Figure S.1" and main "Figure 1" both mapped to "figure_001"),
the second occurrence is renamed with an "s" prefix:
figure_001 -> figure_s001, figure_ss001, etc.
This avoids file overwrite in ocr_objects.py and ensures both
body and supplementary figures appear at their correct positions
in the rendered fulltext.
Changes:
- _resolve_figure_id_collisions() in ocr_figures.py
- figure_id passthrough in ocr_figure_reader.py (reader + normalize)
- _reader_figure_embed_target prefers explicit figure_id
- 11 unit tests covering single/triple/multi collisions, mixed buckets,
empty IDs, realistic body+supplementary layout
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