Research: OpenViking as optional doc backend (integration sketch + tradeoffs)

[aurexav]

Goal\n- Evaluate whether OpenViking should be used as an optional Doc backend (not replacing ELF core), and document an integration sketch.\n\nQuestions\n- What does OpenViking provide that we would otherwise build (hierarchical retrieval, trajectory explain, staged context loading)?\n- What is the blast radius (multi-tenancy, grants, English-only contract, provenance pointers)?\n- How would source_ref point into a viking:// namespace?\n- Operational implications: deployment, upgrades, rebuild semantics, cost.\n\nDeliverables\n- Short decision memo with a recommendation (use / borrow / ignore).\n- If 'use': define an adapter contract + minimal MCP tools for doc hydrate/search.