docs: define SimplicityHL responsibility model

[tvolk131]

Summary

• define validated market creation as the base-case security boundary
• separate collateral safety from permissionless progress
• document transition, static-parameter, and tooling responsibilities
• provide a decision rule for placing checks in SimplicityHL, creation validation, or protocol policy

Why

The binary-market SimplicityHL rewrite needs a stable responsibility model that can guide implementation and review without coupling the design to current function names or path numbering. This also makes clear that recovery and canonical payout restrictions are interoperability policy rather than covenant solvency invariants.

Impact

Documentation only. Runtime behavior and consensus artifacts are unchanged.

Validation

• git diff --check
• GitHub branch comparison confirms one added file and one commit ahead of master

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