Initial v3.1 spec + plan for the delta thickening of v3. v3.1 is the canonical v3 review at depth (>=3,800 LOC main review) with a chunking strategy that v3 lacked. Adds 3 new top-level sections (YAML avoidance, agent context-window, fine-tuning). Load-bearing principle: v3.1 is standalone-readable without consulting v2.3 or v3.
