[Subtask] Visibility mode: hide non-selected nodes and incident edges

[oxy86]

Overview

Add a persistent visibility mode that hides all non-selected nodes and their incident edges, so users can keep working on a reduced on-screen graph without modifying the underlying network.

This differs from a one-shot “focus on selection” action: it behaves as a toggleable view mode that can stay active while the user continues exploring, repositioning nodes, or running layouts on the visible subset.

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Behavior

When enabled:

• Selected nodes remain visible
• All non-selected nodes are hidden
• All edges incident to hidden nodes are hidden

When disabled:

• Full graph visibility is restored

If the selection changes while the mode is active, the visible set should be recomputed.

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UI Suggestions

• Menu / context menu entry:
  • "Hide non-selected nodes"
• Optional checkbox / toggle button in toolbar or side panel

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Implementation Notes

• Must be non-destructive
• Should operate through existing visibility controls rather than deleting data
• Can reuse the same visibility plumbing as:
  • setNodeVisibility(...)
  • setEdgeVisibility(...)
• Works naturally with GraphicsWidget::selectedNodes() and current scene selection APIs :contentReference[oaicite:0]{index=0}

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Edge Cases

• No selection:
  • either do nothing, or
  • show a short user-facing message
• Multi-selection:
  • keep all selected nodes visible
  • keep only edges between visible nodes visible

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Relation to Feature

Part of:
#209 Graph decluttering & focus visualization

[oxy86]

Close #212 as a duplicate of #210.