docs: add layer overview and per-layer scenarios

[heijiLee]

Summary

• Add docs/layers-overview.md: onboarding-oriented summary of the five layers (Token & Identity, Compliance Policy, Execution & Routing, Venue/Adapter, Deployment & Operations) with each layer's core question, rejection point, and the components that own it.
• Add docs/layer-scenarios.md: per-layer success / pre-execution-reject / mid-execution-revert scenarios so newcomers can see what input each layer decides on and where atomic rollback happens.
• Both docs defer to architecture/, MVP-v2-multi-venue.md, and ROADMAP.md as the source of truth — these are reading aids, not specs.

Test plan

• Read both docs top-to-bottom and confirm internal links resolve.
• Confirm layer responsibilities match architecture/README.md.

[heijiLee]

@0xMuang Please make sure my understanding is correct.