paper_plots _extract_bars drops bar-container labels from sidecar point rows

Goal

Fix src/explore_persona_space/analysis/paper_plots.py::_extract_bars so that per-row series labels survive into bar-chart .meta.json sidecars whenever the generator sets ax.bar(label=...). Today the group-level label never reaches the flattened points rows, so labeled bar charts produce sidecars with null/absent series values even though the label exists on the matplotlib bar container.

Provenance

Surfaced by the #2479 analyzer round-2 report (2026-08-24) as the root cause of interpretation-critic concern codex-interp-r1-c11 ("Figure 3 and Figure 5 sidecars omit explicit series labels although their group arrays are hash-distinct"). The #2479 round-2 fix re-annotated generator-side; this task fixes the extractor so labeled bar charts are sidecar-complete without per-generator workarounds.

Design sketch

In _extract_bars, read each bar container's legend label (container.get_label() / the artist's label, skipping matplotlib's auto _nolegend_-style underscore labels) and stamp it as series on every flattened point row derived from that container. Grouped-bar figures with N containers then emit N distinct series values; unlabeled containers keep today's behavior.

Acceptance criteria

1. A figure built with two ax.bar(..., label=...) calls produces a sidecar whose every points row carries the correct non-null series value.
2. An unlabeled single-series bar chart is unchanged (no spurious _nolegend_-style values written as series).
3. A pytest covering both cases lands in tests/ (extend the existing paper_plots sidecar tests if present).
4. Existing sidecar consumers (verify/report tooling) stay green — full relevant test files pass.