Add progressive tabular stats annotations

[jcushman]

Summary

• add an opt-in binoc.tabular_stats_annotator transformer for numeric distribution shifts in tabular artifacts
• add progressive renderer annotations to the SDK/IR as namespaced {package, key, value} records, with Rust and Python helper APIs
• render annotations generically in Markdown by JSON shape instead of adding stats-specific Markdown handling
• publish distribution shifts as simple binoc/distribution_shifts string-list annotations
• add the distribution-shift test vector, regenerate affected ABI/changeset/schema/docs outputs, and document the annotation contract in an ADR

Design notes

• DiffNode::annotate_from(package, key, value) is the explicit annotation writer. The SDK intentionally does not provide annotate(key, value) yet because plugin calls do not have an ambient current-package identity.
• Annotation values stay progressively typed JSON. There is no explicit version field yet; renderers duck-type the generic value shape until a package/key-specific contract is worth stabilizing.
• Markdown intentionally has no special handling for distribution stats. It only suppresses content_summary / tabular_summary from generic output because those are already consumed as move/correlation trailer text.