[M4] Similarity edge threshold slider (default 40% in comparison mode)

[Korede-TA]

Goal

A threshold control that filters which similarity edges are rendered, defaulting to 40% when comparison mode is active (matching legacy archive-knowledge-base behaviour).

What to do

• Add similarityThreshold: number (0–1) to filter-store, default 0
• Toolbar: similarity threshold slider (shown only when similarity edges exist in the manifest)
  • Label: "Similarity ≥ X%"
  • Range: 0–100%, step 5%
• When comparison mode activates (two docs pinned): auto-set threshold to 0.4
• All edge-rendering layers check edge.weight >= filter-store.similarityThreshold:
  • SemanticFrame cross-doc arcs
  • GraphFrame LineLayer (similarity edges only — next/citation edges unaffected)
  • SemanticFrame k-NN overlay

Acceptance criteria

• Moving the slider instantly updates arc visibility across frames
• Entering comparison mode resets slider to 40%
• Slider is hidden when no similarity edges are present in the manifest