[SDK-8] Reference plugin in-repo (canonical example + smoke test)

Ship a canonical example plugin in-tree that doubles as an end-to-end smoke test.

## Scope

- New project under `samples/Fallout.Plugin.Sample/` or similar.
- Exercises at least one surface from each of SDK-6a..6f.
- Wired into the test build: a build runs against a host that loads the sample plugin and verifies the contributions take effect.
- Serves as the worked example referenced by SDK-10 docs.

## Depends on

SDK-2, SDK-3, SDK-6a..6f.