[cravex2-reachability] Enhance the vulnerability-ranking system for trees

[tdruez]

Issues

• Issue: cravex2-reachability: Enhance the vulnerability-ranking system for trees #366

SSVC Decision Triage Rule

• New triage rule SSVCDecisionTriageRule (ssvc_decision), matching vulnerabilities whose SSVC decision tree recommends Attend or Act
• Reference ruleset and preset ("SSVC Attend or Act" / "Flag - SSVC Decision") added to the create_triage_rulesets seeding command
• Vulnerability.highest_ssvc_decision property returning the most severe decision among a vulnerability's published SSVC trees
• Vulnerability info popover now shows the SSVC decision alongside exploitability and risk score
• Reference documentation updated with the new rule