2 — Create shared ontology data composable

Problem: 5 places independently fetch and parse ontology-schema.json (96K) and taxonomies.json (36K):

• HomePage.vue — static imports (132K bundled)
• OntologyBrowser.vue — runtime fetch
• YamlSchemas.vue — runtime fetch
• RelationshipTypes.vue — runtime fetch
• docs/model/index.md — runtime fetch

Action:

• Create .vitepress/data/useOntologyData.ts composable
• Single fetch with cache, typed reactive access
• All consumers use the composable

Files touched:

• .vitepress/data/useOntologyData.ts (new)
• .vitepress/theme/components/HomePage.vue
• .vitepress/theme/components/OntologyBrowser.vue
• .vitepress/theme/components/YamlSchemas.vue
• .vitepress/theme/components/RelationshipTypes.vue
• docs/model/index.md

Done when: One data fetching pattern. No component directly fetches ontology JSON.