docs(pillar): add substrate-tier design charter

Threads the unified thesis through Pillars 12-17:
- Standing-challenge framing (mathematics demands; substrate is on trial)
- Tier architecture (cognitive-architecture 6-11 vs substrate 12-17)
- Per-pillar walkthroughs (claim, why-substrate-needs, probe, criteria)
- Synthetic-only discipline
- Seven-exchange backstory as motivation
- Future candidate pillars 18+
- Open questions and known limits

Author: claude