docs: add the curriculum design doc

Lay out the teaching model (the identity thesis, the 85% fidelity
boundary, the 0-90 five-ladder, the ELI band taxonomy), the enabling
infrastructure (RAM view, schematic engine, deep-lesson surfacing,
predicates, catalog invariants), the full level catalog, the lesson
spec, the schematic inventory, and two worked exemplars (pandas
shipped, asyncio designed). The roadmap maps the remaining 0-90 plan.

Author: alisadeghiaghili