[v0.3] Save and reopen investigation state #34

Tracks #31. Target: v0.3 — Visualization → Investigation.

Problem

Graph exploration is currently transient; investigators need to resume a specific hypothesis and reproduce their view.

Scope

• Save a local investigation containing graph/source reference, saved query, selected seed node IDs, traversal direction/hop/type filters, highlighted paths and current dashboard/view state.
• Distinguish investigation metadata from canonical graph records; avoid modifying the graph to store purely UI state.
• Reopen after app restart; gracefully handle missing IDs, source changes and stale snapshots.
• Store no credentials or secret plugin configuration.

Acceptance criteria

• Saving and reopening restores the selected path and filters on unchanged graph data.
• Changes to the source snapshot are detected and flagged, not silently conflated.
• Versioned serialization with backward-compatible failure handling.
• Works with khonsu CLI / .dash convention where appropriate.
• Tests for reopen, invalid metadata and missing graph path.

Out of scope: multi-user collaboration and cloud sync.