feat: complete Milestones 2–4 — RETRIEVE query, vector index, ranking profiles, diversity, entity system, sessions
M2: RETRIEVE query pipeline with 5-stage execution (candidate → filter → score → diversify → limit),
usearch HNSW vector index, bitmap/range/universe filters, ranking profiles with signal scoring,
MMR diversity enforcement, and m2_uat integration tests.
M3: Entity system with typed metadata, relationship graph (follows/blocks/interactions),
creator entities, session tracking, and m3_uat integration tests.
M4: Advanced ranking with builtin functions (freshness, trending, controversy, wilson),
ranking executor with explain mode, query executor integration, benchmarks for
query/ranking/vector/filters/diversity, and m4_uat integration tests.
Includes: 9 new blog posts, marketing site updates, updated roadmap, and updated vision doc.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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