feat(org-chart): implement expandable layout and multi-company navigation

Integrate P5-1B through P5-4 shared Core layout, collapse, zoom/pan/fit, company roots and keyboard accessibility. Include React/HTML examples, virtual 20/100/300 employee fixtures, focused checks and the additive data contract.

docs/org-chart/data-contract.md

@@ -14,6 +14,7 @@ type RelationType = "cooperation" | "support";
 
 interface Organization {
   id: ID;
+  companyId?: ID; // P5-2/P5-3 Auto 회사 Root 식별. 하위 조직은 상속, 기존 JSON 생략 허용.
   parentId: ID | null;
   name: string;
   type: OrganizationType;