## Summary


- Fix K-means k divergence between Python and Clojure by preserving vote-encounter order for participant rows in the rating matrix
- Python was using `natsorted()` (PID-numeric order) while Clojure's NamedMatrix preserves insertion order — different row ordering cascades into different first-k-distinct initialization seeds for group-level k-means
- On vw: Python picked k=4 (wrong), Clojure picks k=2 — now both pick k=2 with identical cluster memberships

## Investigation findings

The divergence chain: rating_mat row order → PCA projection order → base-cluster ID assignment → group k-means first-k-distinct init → different local optima → different silhouette landscape → different k.

PCA components are identical (cosine similarity = 1.0), silhouette implementation matches, k-means algorithm matches — only the data ORDER feeding first-k-distinct differed.

## Changes

- `conversation.py`: `update_votes()` preserves vote-encounter order for participant rows instead of `natsorted()`
- `conversation.py`: `_apply_moderation()` preserves row order with list comprehension
- Column (comment ID) ordering remains `natsorted` — doesn't affect clustering
- Re-recorded vw cold-start blob and golden snapshots
- Updated ordering tests, removed `test_group_clustering` xfail
- Added `scripts/investigate_k_divergence.py` diagnostic tool

## Cold-start blob results

| Dataset | Clj k | Py k | Match |
|---------|-------|------|-------|
| vw | 2 | 2 | exact (sizes [50,17]) |
| biodiversity | 2 | 2 | exact (sizes [81,19]) |
| bg2018 | 2 | 2 | close ([51,49] vs [52,48]) |
| FLI | 2 | 3 | inherent PCA divergence (94.5% NaN, sil gap 0.001) |

## Test plan

- [x] All 297 tests pass (0 failures, 58 xfailed)
- [x] vw cold-start: k=2 exact match with Clojure blob
- [x] biodiversity cold-start: k=2 exact match
- [x] Ordering tests updated to expect encounter order
- [ ] Re-record private dataset golden snapshots after stack rebase

🤖 Generated with [Claude Code](https://claude.com/claude-code)


## Squashed commits

- Fix K-means k divergence: preserve vote-encounter order for participant rows
- Update plan and journal: K-divergence investigation resolved
- Remove investigation script (one-off diagnostic, not production code)
- Rename k-divergence doc: investigation record, not a handoff
- Update references to renamed investigation doc

commit-id:4598a0a1