docs: add ADR-268 and research doc for semantic-cache

ADR-268: proposes ruvector-semantic-cache as first-class RuVector
capability with SemanticCache trait, 3 variants, benchmark evidence
(13.49x speedup), failure modes, security considerations, migration path.

Research doc covers:
- 2026 SOTA survey (QVCache, vCache, GPTCache, CacheRAG, Bifrost)
- Forward-looking 2036-2046 thesis (semantic manifolds for agents)
- ruvnet ecosystem fit (agent-memory, lsm-ann, proof-gate, ruFlo, MCP)
- Real benchmark results
- Memory and performance math
- 8 practical + 8 exotic applications
- Production crate layout proposal
- ADR-268

Co-Authored-By: claude-flow <ruv@ruv.net>
Claude-Session: https://claude.ai/code/session_01FW9sGTp6EzHqbyxKhvAG49

Author: claude