xbrain diff — compare two snapshots, surface unexpected drift

[VGonPa]

After a re-enrichment, it is currently impossible to tell how much changed. Did 2% of items get reassigned, or 40%? Did the topic-page overviews shift drastically? Today: no diff, no answer.

Depends on: snapshot system (#C).

What drift means

Two distinct phenomena that the diff distinguishes:

1. Expected drift from corpus growth — a topic with +20 items (from 50 to 70) should change its overview. Healthy signal.
2. Unexpected drift from prompt/model change — same items reassigned to different topics, or an overview rewritten when only 2 items changed. Noise.

The diff surfaces both; the user (or eval, #8) judges which is which.

Mini-spec

`xbrain diff ` (default `snapshot-b` = current live state).

Output sections:

• Items reassigned: count + % of items whose `primary_topic` changed. List the top N most-frequent transitions (e.g. `ai-coding → software-engineering: 12 items`).
• Topic-level changes: for each topic, items added / removed / unchanged. Flag topics with >10% growth or >10% shrinkage.
• Overview drift: for each topic, similarity between old and new overview (cosine similarity of embeddings, or LLM-judged similarity if WS3 WS3 — enrichment evaluation harness #8 is available). Flag overviews that changed sharply on small corpus changes.
• Vocab changes: which slugs were added, removed, renamed.

Optionally `--format json` for machine consumption (CI / WS3 eval can ingest).

Acceptance

• Works without WS3 / LLM judge: minimum viable diff is mechanical (counts, set differences, embedding similarity via a small offline model).
• Adds an optional `--judge` flag that uses the WS3 judge if WS3 — enrichment evaluation harness #8 is built.
• Tests cover: snapshot diff with fixture pairs; transitions correctly counted; vocab changes detected.

Why this is not part of WS3

WS3 compares against a fixed gold standard ("is your enrichment good in absolute terms?"). `xbrain diff` compares two runs of your system ("is your enrichment stable across changes?"). Different question, different tool, shared infrastructure.

[VGonPa]

Closed by #28 (merged to develop).