[M4] SemanticFrame: cross-document similarity arcs in comparison mode

[Korede-TA]

Goal

When two documents are pinned for comparison, visualise cross-document chunk similarity as arcs in SemanticFrame, filtered by the similarity threshold (see similarity threshold issue).

What to do

• Detect comparison mode: view-store has two pinned doc IDs
• Filter data-store.edges for type === 'similarity' edges where source and target belong to different pinned docs
• Render as LineLayer (or ArcLayer with slight elevation for visibility) with color encoding by weight:
  • Low similarity: dim gray
  • High similarity: accent indigo (#6366f1) → white at 1.0
• Line width proportional to weight
• Only render edges above filter-store.similarityThreshold
• Toggle button in SemanticFrame toolbar: "Show cross-doc arcs"

Acceptance criteria

• Arcs appear between chunk pairs from the two pinned documents
• Color/width encodes similarity strength
• Threshold slider (see [M5] akb_adapter: emit title and source as node properties #44) hides/shows arcs dynamically