docs(memory): add MEMORY_SYSTEM_OVERVIEW.md study writeup

End-to-end walkthrough of the memory stack: ingest, recall, read, decay, consolidation, multimodal, the classifier-driven dispatch pipeline (IngestRouter, QueryClassifier, MemoryRouter, ReaderRouter, ReadRouter), canonical-hybrid retrieval (BM25 + dense + Cohere rerank-v3.5), the eight cognitive mechanisms, the seven-step consolidation loop, and the storage substrate via @framers/sql-storage-adapter (SQLite, Postgres, IndexedDB, Capacitor, Electron, in-memory).

Decomposes the +9 pp lift to 85.6% on LongMemEval-S, the 70.2% on LongMemEval-M, the cost-Pareto improvements, the negative findings (Stage L, Stage I, hierarchical retrieval, etc.), the vendor comparison with caveats, and the single-CLI reproduction recipe.

Author: jddunn