Add CI contract checks for the merged plugin artifact

[kelchm]

Motivation

CI builds the shipped single-DLL artifact with ILRepack, folding FanaBridge.Core and FanaBridge.Updater into merged/FanaBridge.dll. The existing test step still runs against the separate development assemblies.

Merge-sensitive behavior—type identity, BAML resource keys, and embedded-resource manifest names—therefore has no repeatable CI protection. These surfaces were verified manually for #103.

Scope

Add a post-merge contract check that runs in a separate process and loads the exact merged FanaBridge.dll produced by MergePlugin.

Keep this as a focused artifact-level smoke check rather than duplicating the normal unit suite. It should cover the merged plugin’s essential loading contracts:

• The plugin entry type, FanaBridge.FanatecPlugin, is present.
• Built-in wheel profiles can be enumerated and loaded.
• Expected BAML resource keys are present.
• Representative WPF controls can be constructed on an STA thread.

Wire the check into .github/workflows/build.yml after the merge step so packaging cannot proceed with a structurally broken artifact.

Acceptance criteria

• A repeatable contract-check executable or equivalent harness loads the merged FanaBridge.dll in a separate process.
• The check validates the plugin entry type, built-in profiles, BAML resources, and representative WPF construction.
• The check exercises the merged artifact rather than the separate development assemblies.
• CI runs the check after MergePlugin produces the merged DLL.
• A contract failure fails the CI job with actionable diagnostics.
• The existing development-assembly test suite continues to run normally.