feat(graph): Add Investigation Trail and Minimap foundations

Investigation Trail System (Complete):
- InvestigationTrail struct with breadcrumb navigation
- TrailStep tracking (wallet, risk, timestamp, notes)
- Back/forward navigation (browser-style history)
- Fork logic (truncates forward history on branch)
- Export summary with risk color coding
- Auto-update on node selection
- Node risk calculation (Critical/High/Medium/Low)
- Contextual notes (mixer detection, cluster membership)

Minimap Infrastructure (Ready for rendering):
- Graph fields added (show_minimap, show_trail)
- Architecture designed for corner overlay
- Viewport tracking prepared

Implementation Guide:
- GRAPH_ENHANCEMENTS.md (350+ lines)
- Complete rendering code samples
- Keybinding specifications
- Integration patterns with forensics features

New Methods:
- calculate_node_risk_level() - Quick heuristic risk assessment
- update_trail_on_selection() - Auto-track navigation
- InvestigationTrail::new(), add_step(), go_back(), go_forward()
- export_summary() - Generate investigation narrative

Benefits:
- Transforms exploration into storytelling
- Forensically defensible breadcrumb trail
- Easy backtracking and alternate path exploration
- Export-ready investigation reports
- Non-technical stakeholder communication

Status: Data structures + logic complete (150+ lines)
Next: Rendering implementation (2-3 hours for visual layer)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 0xrinegade