docs: the verification gate cannot terminate, and I left two runs going 4h

Two findings from checking whether last iteration's partial green had
finished. It had not, and the reason matters for anyone using the gate.

build-reborn test contains twin_pairs_do_not_share_a_buffer, which
decodes every .xpr in hidden/resource3d -- 166 files, 1.4 GB -- through
the full Xbg7Model anchoring path, and is NOT #[ignore]d. Its sibling in
the same file walks the same 166 files and IS ignored as known-failing,
which makes the binary's cost easy to underestimate. Measured: one
instance accumulated 3h26m of CPU at 89% without finishing.

So every "green run" reported in this corpus from a workspace or
sylpheed-formats test is necessarily PARTIAL unless it says the suite
terminated -- including the ones I reported this session. The honest form
is the suite count and elapsed state, not the word "green". Not proposing
to #[ignore] or subsample it: that changes what the suite asserts and is
the project's call, not an audit side-effect.

And the mess is mine. Two cargo test -p sylpheed-formats runs launched
detached in earlier iterations never exited, because they were sitting in
that test:

  pid 103375  4h12m elapsed  child mesh_consistency_disc  3h26m CPU  89.3%
  pid  99965  4h39m elapsed  child pak_idxd_disc          1h16m CPU  93.8%

Load average 14.18 on 12 cores. Killed, after checking the legitimately
running workspace suite and leaving it alone; load fell to 9.68.

What this does NOT explain, because it is tempting: the session's
emulator troubles. screenshot cost 0.49 s with both runaways live and the
emulator stopped, against 10.8 s measured earlier with the emulator
running. The 92x figure really was emulator contention; the runaways were
a background tax on top. The black surface and the unreachable title
stand as measured, with their own controls.

METHOD: a detached job you never check can outlive many iterations --
setsid was added so a timeout could not kill them, which also means
nothing does; and know whether your verification gate can terminate.

docs/re/METHOD.md

@@ -721,3 +721,16 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   version grepped for the doc's formatting (`14 709` with a thin space, `33.7`
   rounded from `33.66`) and reported almost every figure as a mismatch. A
   consistency check that fails on formatting will train you to ignore it.
+* **A detached job you never check can outlive many iterations.** Two
+  `setsid nohup cargo test … &` runs from earlier iterations were still alive
+  **four hours** later, one child at 89 % CPU for 3 h 26 m, holding the load
+  average at 14 on 12 cores. `setsid` was added precisely so a tool-call timeout
+  could not kill them — which also means nothing kills them. Check that a
+  backgrounded run actually exited before launching the next one, and prefer
+  `ps -o etime=` over assuming.
+* **Know whether your verification gate can terminate.** `build-reborn test`
+  includes a disc test that decodes 166 `.xpr` files (1.4 GB) and had not
+  finished after 3 h 26 m of CPU. Every "green run" from it is partial unless the
+  suite is stated to have terminated — so report the suite count and elapsed
+  state rather than the word "green", which is what I should have been doing all
+  session.