Add _infer_missing_main_figure_numbers() that fills figure_number=None
for matched main-sequence figures when a single leading gap (Figure 1)
can be inferred with high confidence.
- _FRONTMATTER_VISUAL_VETO + _has_frontmatter_visual_veto()
- _FIGURE_MARKER_PATTERN regex + _extract_figure_marker()
- _coerce_int_figure_number(), _resolve_legend_bbox()
- _infer_missing_main_figure_numbers() wired into build_figure_inventory()
- 9 test cases covering acceptance, veto, isolation, skip reasons
- Fix golden vault test key drift (aliases, ocr_time, etc.)
Updates all tests that assert on status --json output to read from PFResult envelope.
Code was intentionally changed in Phase 57 to wrap JSON output in PFResult {ok, command, version, data, error}. Tests must validate the new contract shape, not the old flat dict format.
