docs(arch): add P-038 transition-witness overlay contract

[PhysShell]

What changed

Adds the OwnAudit-side consuming section for the ratified P-038 experiment.

The section fixes the downstream contract for static proxy and later runtime evidence, preserves runtime-only/ambiguous evidence, scopes the primary estimand, and records preregistration requirements.

Scope guard

This is an experimental reporting overlay only. It does not add ARCH codes, change architecture gate verdicts, alter rules/baselines, add acceptance criteria to the architecture-review MVP, or introduce a runtime collector.

Validation

Final diff against main is surgical: docs/architecture-review.md only, +23 / -0. Existing architecture-review wording is otherwise unchanged.

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