NativeAOT mock-VISA container image

[ShortArrow]

Context

The current ghcr.io/shortarrow/ivi-cli-mock image bundles a
self-contained .NET runtime (~80 MB compressed). A NativeAOT
build would yield a substantially smaller image with faster
cold start — attractive for e2e tests that spin up a fresh
container per scenario.

ADR 0018 §Out-of-scope
deferred this pending an AOT-compat audit.

Dependencies needing AOT audit

• Serilog + sinks (file / console / Compact JSON formatter)
• OpenTelemetry SDK + Exporter.OpenTelemetryProtocol +
  Instrumentation.AspNetCore
• ASP.NET Core 10 minimal API (for ivicli api start; not used
  inside the mock-only container, but the binary still references
  it)
• Tomlyn
• Plugin loader: AssemblyLoadContext + reflection-based discovery
  (ADR 0013)

Acceptance

• AOT-compat audit completed; tracked decisions per dependency.
• IVICLI_MOCK_ONLY=1 build path can publish AOT.
• Container image size drops to <30 MB compressed.
• Smoke test (existing pr-docker-smoke.yml) passes against the
  AOT image.