Static embed should support diff/compare view

[paddymul]

Context

The static embed (static-embed.js / to_html()) currently supports two embed types: DFViewer (plain table) and Buckaroo (full widget with summary stats switcher).

The xorq-mcp web UI has a diff/compare view that shows two DataFrames side-by-side using Buckaroo's compare functionality (via the live Buckaroo server as an iframe). This is useful for comparing two revisions of a catalog entry.

Request

Add a Compare or Diff embed type to the static embed pipeline so that two DataFrames can be compared without a running Buckaroo server.

The workflow would be:

from buckaroo.artifact import prepare_compare_artifact, artifact_to_json

artifact = prepare_compare_artifact(df_old, df_new, join_key="id")
json_str = artifact_to_json(artifact)
# -> renders a side-by-side diff table via static-embed.js

This would allow the xorq-cloud catalog viewer (which uses pre-rendered static artifacts, not a live Buckaroo server) to support diff pages.

Current workaround

The cloud viewer has no diff view — it's listed as "not built" because the static embed doesn't support it.