feat(dotnet): model release evidence

[enricopiovesan]

Summary

• add typed .NET release evidence for package/runtime traceability
• require package version, runtime-WASM digest, conformance version, and host versions
• reject incomplete evidence before publication

Governing Spec

• 068-public-platform-embedder-packages

Project Item

Refs #649

Definition of Done

• .NET exposes the release-evidence fields required by FR-008.
• Incomplete evidence is rejected at the public package boundary.
• Evidence publication, production runtime bridge, and WinUI integration remain tracked by Windows: ship public .NET embedder SDK for WinUI clients #649.

Validation

• git diff --check
• The .NET SDK is unavailable locally; CI compiles the package.