Add runnable smoke-test clients for each SDK

Each package gets a small example program that exercises the real
client: with no ORIGIN_SERVICE_KEY set it runs against a local mock
check-in server, otherwise it checks in against ORIGIN_URL/Origin's
public endpoint. Wired into CI (mock mode) as a regression check, and
documented per README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

rust/Cargo.toml

@@ -22,4 +22,4 @@ winreg = "0.52"
 
 [dev-dependencies]
 mockito = "1"
-tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time", "signal"] }