StremioServer::start() has no idempotency guard

[kKaskak]

Calling start() twice spawns a second stremio-runtime child and job object. While currently only called once from build, the crash-restart path at server.rs:235 re-calls it without checking if the previous child is still alive. Should guard with an "already running" flag.

[kKaskak]

Verdict: valid.

Reviewed against current main: StremioServer::start() has no running/in-flight state and each call spawns a new thread/runtime before blocking on readiness. Startup and crash notice are current callers; repeated notices or future calls can create overlapping runtimes. Add a guard and clear it when child lifecycle ends.