feat: add plot-batch manifest and shared plot data loader

[claytonlin1110]

Summary

Adds paperbanana plot-batch so multiple statistical plots (CSV/JSON + intent) can run in one batch with the same outputs/batch_* layout and batch_report.json as diagram batches. Studio Batch tab can run plot manifests. Diagram batches now set batch_kind: methodology; plot batches set statistical_plot. batch-report MD/HTML shows kind when present.

Closes #122

Motivation

Authors often need many data figures per paper; looping plot manually loses a unified report and output tree. This aligns plots with the existing batch workflow.

How to test

• pytest tests/test_batch.py tests/test_core/test_plot_data.py
• paperbanana plot-batch --manifest examples/plot_batch_manifest.yaml (requires API keys)
• paperbanana batch-report --batch-dir <batch_dir> --format markdown

[claytonlin1110]

@dippatel1994 Any update about this feature?

[claytonlin1110]

@dippatel1994 Please review

[dippatel1994]

CI passes, clean extraction of shared data loader. One behavioral concern:

1. JSON data unwrapping regression — Old Studio run_plot had raw_data = raw if isinstance(raw, list) else raw.get("data", raw) which unwrapped a {"data": [...]} envelope. New load_statistical_plot_payload() returns raw JSON as-is, then callers wrap as raw_data={"data": payload}. If a user passes {"data": [{"x":1}]}, old code produces raw_data={"data": [...]} (unwrapped), new code produces raw_data={"data": {"data": [...]}} (double-wrapped). Decide which behavior is correct and be consistent.

Non-blocking: total_seconds missing from Studio run_plot_batch report (always shows 0.0s). Pre-existing pattern but easy to fix here. Also no test for empty manifest handling.

[claytonlin1110]

@dippatel1994 Just updated.

[dippatel1994]

JSON unwrapping regression fixed, total_seconds added to Studio report, empty manifest test added. CI green. LGTM.

[claytonlin1110]

Thanks @dippatel1994
ready to be merged?