Update README — stale architecture section

[StigNorland]

README describes an earlier design (nanoGPT KV-cache replacement; src/graph/, src/memory/, src/model/) that no longer exists. The real system is a LoCoMo-evaluated conversational memory pipeline in src/library/.

• Rewrite Project Structure + Quick Start to match the real code
• Reframe the holographic field honestly (compression tool, not recovery — per its own eval)
• Point to MEMORY.md for architecture-as-built

See MEMORY.md.