[Subtask] Export or save the current filtered graph / visible subset

[oxy86]

Overview

After users apply node and edge filters, they need a way to persist the currently visible subset of the graph.

This subtask adds support for exporting or saving the current filtered/visible graph as a standalone graph dataset.

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Motivation

Filtering becomes much more useful when users can:

• preserve the visible subset
• analyze it later
• share it
• continue working on it as an independent graph

This makes filtering a complete workflow rather than a temporary visualization aid only.

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Scope

Allow users to save/export the currently visible subset of the graph.

The exported graph should include:

• all currently visible nodes
• all currently visible edges between them
• node and edge attributes, where supported by the chosen format

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Behavior

• The original graph remains unchanged
• Export acts on the current visible state only
• Result is a new graph file representing the filtered subset

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UI Suggestions

• Menu action:
  • "Export visible graph"
  • or "Save visible subset as..."
• Optional confirmation dialog showing:
  • visible node count
  • visible edge count

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Implementation Notes

• Must work with the current single-window workflow
• Should reuse existing export/save infrastructure where possible
• Needs clear behavior when hidden nodes/edges exist
• Document format-specific limitations if some exports do not preserve all attributes

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Dependencies

• [Subtask] Unify and extend existing node filtering by structural metrics #216 Structural filtering
• [Subtask] Filter nodes and edges by attributes (metadata-based filtering) #217 Attribute filtering
• [Subtask] Unify node and edge filtering into a single filtering system #219 Unified filtering system
• Existing save/export pipeline

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Edge Cases

• No active filters, full graph visible
• Hidden nodes but visible edges impossible by invariant
• Multi-relational graphs
• Format-specific attribute loss

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Relation to Feature

Part of:
#215 Graph filtering, querying & subgraph extraction