[FEAT] Show comments/descriptions in branch diff view

[divideby0]

Summary

When diffing between branches, surface any comments/notes that were added or changed alongside the rating changes.

Motivation

The current diff view (PR #16) shows rating value changes between branches. But if users add comments explaining why they changed a rating, those comments should be visible in the diff too — otherwise you see the "what" but not the "why."

Suggested Approach

• If cells have comments/notes (from [FEAT] Add markdown descriptions to criteria and options #9 descriptions or a future comment system), include them in the diff view
• Changed comments could appear inline below the cell or in a tooltip/popover
• Added/removed comments should be visually distinct from rating changes

Priority

Low — depends on the comment/description system from #9 being in place first.

Context

From Slack discussion — Cedric asked: "do we also show comments in the diff?"

[evie-assist]

Implementation Notes

Based on review of feat/branch-diff-merge-ui at commit c5083db.

Comments already exist in the event model

ScoreSet already has an optional comment field (line 47):

export interface ScoreSet extends PughEventBase {
  type: 'ScoreSet';
  toolId: string;
  criterionId: string;
  score?: number;
  label?: string;
  comment?: string;  // ← already here
}

And the store's PughUIState already tracks editComment for the cell editing flow. So comments are being captured.

Current diff ignores comments

describeEvent() in BranchSelector.tsx renders events in the diff panel but ignores the comment field entirely (line 19-20):

case 'ScoreSet':
  return event.label ? `Set score: ${event.label}` : event.score != null ? `Set score: ${event.score}` : 'Set score';
  // ← no mention of comment

The diff panel itself (lines 229-265) iterates over branchDiff.sourceEvents and targetEvents, rendering each via describeEvent().

Minimal fix (~5 lines)

Update describeEvent() at line 19:

case 'ScoreSet': {
  const parts: string[] = [];
  if (event.label) parts.push(event.label);
  else if (event.score != null) parts.push(`${event.score}`);
  if (event.comment) parts.push(`"${event.comment}"`);
  return `Set score: ${parts.join(' — ') || '(cleared)'}`;
}

Richer approach: cell-level diff

The BranchDiff type already includes sourceState and targetState as PughDomainState. A cell-level diff view could compare projected scores side-by-side, showing before/after score and comment for each changed cell.

Worth checking if projectEvents() preserves the comment field through to the projected ScoreEntry — if not, that's the first thing to fix.

Dependency on #9

Issue #9 (markdown descriptions for criteria/options) would add structural descriptions on criteria and options themselves. If those get their own event types (e.g., CriterionDescriptionSet), describeEvent() would need updating too. Worth coordinating.

Scope

• Minimal (update describeEvent): ~5 lines
• Cell-level diff view: ~100-150 lines (new component + CSS)