What

Adds docs/security-model.md, a document describing the SDK's security model. The repository previously had no SDK-level security/trust-model documentation — only SECURITY.md (a vulnerability-reporting policy) and a per-demo "Security Assumptions" section in eth_wallet.

Why

TrustZone development differs from ordinary application development: code is partitioned into an untrusted Normal World and a trusted Secure World, and the boundary between them is the central security property. A single document that maps this trust model onto the codebase helps both human developers and automated/LLM security reviewers reason correctly — in particular, to focus on the real trust boundary and avoid false positives on code where the concern does not apply.

Contents

docs/security-model.md covers:

1. Trust model — TCB, adversary capabilities (full Normal-World root: arbitrary parameters, attacker-chosen buffer pointers/lengths, concurrent mutation of shared memory), and out-of-scope threats.
2. The trust boundary — the TA entry point; per-parameter-type trust analysis (Value* vs. shared-memory Memref*); and the invariants TA boundary code must enforce (validate param_types, treat every memref byte as adversarial, bound lengths, copy-then-validate to avoid TOCTOU/double-fetch, don't leak via output buffers, fail closed).
3. Trust-posture map of the repository — a table mapping each path to its world/role and what to scrutinize.
4. Storage, secrets, and other assumptions — secure-storage confidentiality vs. rollback/availability, RPMB, secure UI.
5. Dependencies and the supply chain — a TA's full transitive crate tree runs inside the TEE and is part of the TCB; no-std vs std as a TCB-size decision; software crypto and entropy considerations; build-time supply-chain risk; and vetting recommendations.
6. Guidance for automated / LLM security review — where real findings concentrate, expected non-findings (false positives), and an explicit instruction to also review the TA's dependency tree when tooling allows.

Also links the new doc from SECURITY.md and the docs index (docs/README.md).

This is documentation only; no code or behavior changes.

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