A redirected log lags; do not diagnose a stall from it

Node block-buffers stdout to a file, so a run log can sit unchanged for
minutes while the run is fine. Read that way twice today — the second
time straight after fixing a real stall, which is the worst version of
it, because a buffering artifact then reads as the fix having failed.

The machines are the source of truth and answer immediately. Written
down with the two commands that settle it.

README.md

@@ -165,6 +165,19 @@ If it says the daemon is not reachable, the group grant postdates the shell. `ne
 but a heredoc into `newgrp` runs the suite as a child of a shell that then exits — start it with
 `setsid nohup … &` inside the heredoc, or the run dies with the shell that launched it.
 
+**A redirected log lags, so do not diagnose a stall from it.** Node block-buffers stdout when it
+is a file rather than a terminal, so `> run.log` can sit unchanged for minutes while the run is
+working normally. On 2026-09-01 that was read as a stall twice, once after a real stall had just
+been fixed — the most expensive kind of false signal, because it argues the fix did not work. Ask
+the machines instead:
+
+```sh
+incus list -c ns
+incus exec <instance>-registry -- systemctl is-active docker
+```
+
+*"I cannot see progress" is not evidence of no progress.*
+
 ## Measured on a workstation
 
 | | one machine | two machines | two machines + a router |