paper_plots._extract_scatters drops genuine (0,0) single-point scatters from figure sidecars

target_file: src/explore_persona_space/analysis/paper_plots.py problem: _extract_scatters skips ANY single-point PathCollection whose lone offset is exactly (0,0) as "matplotlib's default for empty collections" (lines 852-854). Per-point ax.scatter([x],[y]) loops — the standard per-carrier/per-pair companion pattern — therefore silently drop every genuine (0,0) datum from the sidecar points AND total_points (issue 2162 ladder_percarrier: 252 captured vs 264 rendered), corrupting the exact surface report-verifier recomputes against and nearly producing a spurious FAIL in the #2162 ladder round (report-verified round 1, 2026-08-13). fix_sketch: discriminate a real point from the empty-collection default via artist properties instead of coordinates — e.g. skip only when offsets.shape[0]==1 and not np.any(offsets) and coll.get_sizes().size == 0 (a real scatter(...,s=...) carries a nonempty sizes array), or when facecolors are unset; add a regression test rendering ax.scatter([0.0],[0.0], s=14) and asserting the sidecar captures it. confidence: high