docs(wiki): add Validation and Reasoning page; link from General Concepts Captures the three-layer boundary (storage / validation / inference), reviews candidate languages (SHACL, ShEx, PG Schema, JSON Schema, in-graph shapes), and lists the open design questions to resolve when the Graph API is designed. No implementation decision made yet. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>