LESSONS: thermal hard-freeze pattern and diagnosis by exclusion

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

LESSONS.md

@@ -62,7 +62,15 @@ reproduction: [`RESULTS.md`](RESULTS.md). Everything on `google/gemma-4-E2B-it`
 6. **Background jobs must be `setsid`'d** or the harness/session restart
    kills them mid-run. And `pkill -f <pattern>` will match your own launcher
    shell if the pattern appears in its command line.
-7. **Zero-init adapter output layer ⇒ zero grads upstream at step 0** — on
+7. **Sustained training in a closed cabinet = thermal hard-freezes.** Four
+   crashes in one day (journal stops mid-line, no OOM, no shutdown trace,
+   37GB free at one death) on a DGX Spark that was stable all week under
+   light load. Pattern: only under hours of continuous GPU load; fixed by
+   opening the cabinet. Diagnose by exclusion: earlyoom quiet + journal
+   truncation + load correlation = thermal, not software. And do NOT
+   auto-restart training via @reboot cron on a thermally-suspect box — it
+   risks a crash loop with no human circuit breaker.
+8. **Zero-init adapter output layer ⇒ zero grads upstream at step 0** — on
    `mlp[0]` this is expected (LoRA-B-style), not a bug; check the output
    layer's grad instead.