feat: add interactive retrieval learning course

[jameswei]

Summary

• add a six-module interactive retrieval course to the web Studio
• visualize lexical, dense, hybrid, reranking, evaluation, and optional Qdrant workflows
• bundle the reviewed browser evaluation set and refresh the public README, landing page, and learning guides
• keep full and slim images CPU-only with explicit model lifecycle controls

Verification

• 855 passed, 8 skipped
• 24 web tests passed
• web and Learning Guides production builds passed
• full, slim, and Qdrant smoke checks passed
• Phase 3.4 manual preview and independent review signed off