variant and ClinVar boundary fixes
- harden variant and ClinVar boundary handling across the tool and resource surface\n- add ambiguous rsID candidates, gnomAD attribution, and graceful optional ClinVar degradation\n- follow-up issues: #11 #12 #13 #14 #15 #16 #17 #18 #19 #20 #22\n- changelog: CHANGELOG.md
