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/README.md

@@ -54,6 +54,17 @@ default. `Config` implements `Default`, so use struct-update syntax
 `resolve_machine_id()` are exported standalone for callers that want the
 detection logic without the HTTP client.
 
+## Manual smoke test
+
+```bash
+cargo run --example testclient             # single check-in, local mock server
+cargo run --example testclient -- --watch  # loops via run() until Ctrl-C
+```
+
+Set `ORIGIN_SERVICE_KEY` (and optionally `ORIGIN_URL`, `ORIGIN_TYPE`,
+`ORIGIN_VERSION`, `ORIGIN_NAME`, `ORIGIN_SITE`, `ORIGIN_ENVIRONMENT`,
+`ORIGIN_INTERVAL_HOURS`) to check in against a real endpoint instead.
+
 ## Platform notes
 
 macOS/Windows machine-ID detection is compiled but not exercised by CI