Validate realtime plot summary shape

[sylvesterkaczmarek]

Summary

• Require summary.json loaded by the realtime plotting CLI to contain a JSON object.
• Keep valid summary objects and plotting behavior unchanged.

Motivation

plot_eval_results.py currently accepts any syntactically valid JSON value and passes it directly to build_realtime_eval_plots(). A list, string, number, or other non-object JSON value therefore gets past the file-loading boundary and fails later inside plotting code that expects mapping-style summary data.

The run summary contract is an object. Validating that shape immediately produces a clear error at the source instead of an unrelated downstream attribute/type failure.

Validation

• object-valued summary.json -> existing plotting path unchanged
• malformed JSON -> existing JSONDecodeError behavior unchanged
• valid non-object JSON -> clear ValueError naming the summary file

Self-review

• Two added lines in one CLI file.
• No chart behavior changes for valid runs.
• No dependencies, notebooks, registry entries, or documentation changed.
• Searched open PRs for an existing realtime plot summary-shape fix and found none.

Maintainers may modify the branch if needed.