Add GraphCommunityChurnAdvisor: agentic community-evolution churn advisor

Sibling to GraphAdversaryForecaster/GraphPrivacyExposureAuditor/GraphInfluenceSeedAdvisor/GraphCascadingFailureAdvisor/GraphIntelligenceAdvisor. Compares baseline vs current graph snapshots and labels nodes STABLE/MIGRANT/BRIDGE_FORMING/ISOLATING/NEW_ARRIVAL/DEPARTED/CHURNING with 0-100 churn score and P0-P3 priority. Detects community evolution patterns (STABLE/SPLIT/MERGED_INTO/DISSOLVED/SHRUNK/GREW), emits A-F portfolio grade + P0-first deduped community-management playbook (REACH_OUT_ISOLATING/STABILIZE_CHURNING/INVESTIGATE_DISSOLVED_GROUPS/EXIT_INTERVIEW_HIGH_DEGREE_DEPARTED/REVIEW_COMMUNITY_SPLITS/AMPLIFY_BRIDGE_BUILDERS/STUDY_MIGRATION_FLOWS/ONBOARD_NEW_ARRIVALS) + insights (NET_SHRINK/NET_GROWTH/HIGH_STABILITY/CHURN_CLUSTER/BRIDGE_BUILDERS_PRESENT/HIGH_MIGRATION/COMMUNITY_DISSOLUTION/MULTI_COMMUNITY_SPLIT). Risk_appetite CAUTIOUS/BALANCED/AGGRESSIVE modulates scores monotonically. text/markdown/deterministic-JSON renderers. Pure JDK + JUNG, single file, never mutates inputs, deterministic with fixed Clock. 10 passing JUnit 4 tests.

Author: zalenix-builder