tests: the L1 witness may be gone before its teardown; and a correction

The previous commit's message says "Elixir green"; the run it cited had
one failure, which the builder read only after committing (tablet 35's
lesson, repeated). The failure: tablet38_l1_integrity_test's on_exit
stopped a witness linked to the test process after it had already
exited (seed 221974). The teardown now tolerates that. Full suite with
that seed: 607 tests, 0 failures, 46 excluded

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

vav_pipeline/test/tablet38_l1_integrity_test.exs

@@ -172,7 +172,15 @@ defmodule VavPipeline.Tablet38L1IntegrityTest do
         L1Integrity.start_link(l1_root: root, expected_hash: L1Reader.expected_hash(), interval: :never)
 
       on_exit(fn ->
+        # The witness is linked to the test process and may already be
+        # gone when on_exit runs (seed 221974, 2026-10-15): stopping a
+        # process that has exited is not a failure of the scroll.
+        try do
           if Process.alive?(pid), do: GenServer.stop(pid)
+        catch
+          :exit, _ -> :ok
+        end
+
         {:ok, _} = Supervisor.restart_child(VavPipeline.Supervisor, L1Integrity)
         MannaLedger.reset()
         File.rm(VavPipeline.Manna.provision_file())