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

[ChrisonSimtian]

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.