The drain task previously used tokio::spawn + tokio::time::sleep,
gated by Handle::try_current().is_ok(). alpha_train's main() is
synchronous (not #[tokio::main]) so Handle::try_current() returned
Err and the drain task was silently skipped — the trace file was
never created.
Refactor to std::thread::spawn + std::thread::sleep. No runtime
required; spawn is unconditional; Drop joins synchronously instead
of aborting; BufWriter flushes on every drain cycle so even short
runs (< 1 sec) capture their tail. Existing gpu_log_ring_invariants
tests migrated from #[tokio::test] back to plain #[test].
Also resolves an exit-1 issue on alpha_train shutdown — likely a
tokio task abort panicking through the synchronous Drop path.
Local smoke (alpha_train --kernel-step-trace ...):
- EXIT=0
- step_trace.jsonl: 1497 lines (~500 steps × 3 smoothness records)
- valid JSONL, schema matches smoothness_controller emission contract
cd0fe8549c