Smoke-test release images on amd64 and arm64

[ocularminds]

Context

Release images are published for linux/amd64 and linux/arm64, while runtime smoke tests currently exercise only the hosted runner architecture.

Scope

Add architecture-specific runtime checks for the extension, MCP, and authority release candidates.

Acceptance criteria

• The MCP and authority images start and pass their smoke tests on amd64 and arm64.
• The extension image metadata and backend entrypoint are checked on both architectures.
• The checks run before the corresponding multi-platform manifest is published.
• An architecture-specific failure identifies the image, platform, and failing assertion.
• Emulation limitations and any accepted exceptions are documented.