L1Integrity: the ten-minute re-reading runs beside the witness, not in it

On the production node a reading of the scroll takes about three
seconds (3310 ms at first start; 0.9 s on the build host). The periodic
re-reading ran inside the GenServer, so once every ten minutes every
cycle and every readiness probe that asked "is L1 intact?" would have
waited those seconds behind it.

The periodic reading is now done by a task that reports back; the
process goes on answering from its last reading meanwhile. The first
reading is still made in the process itself — nobody is answered before
the scroll has been read once — and an explicit recheck/0 is still
synchronous. The next reading is scheduled when one begins, not when it
reports, so a reading that dies silently is not the last.

Two tests: it re-reads on its own in both directions (altered → fail,
restored → tov), and a status call during a periodic reading returns at
once. 484 tests, 0 failures on seeds 0 7 42 99.

Built image, run with a live-mounted scroll: l1_hash pass, /peah/ask
200; then אחד in Devarim 6:4 was changed to אחר while the vessel ran
and the witness asked to read again — L1_INTEGRITY_FAILED, l1_hash
fail, /peah/ask 503.

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

vav_pipeline/test/tablet38_l1_integrity_test.exs

@@ -126,6 +126,27 @@ defmodule VavPipeline.Tablet38L1IntegrityTest do
       File.write!(Path.join([root, "torah", "index.json"]), "INDEX, altered")
 
       assert eventually(fn -> match?({:error, _}, L1Integrity.status(:tablet38_periodic)) end)
+
+      # And it keeps re-reading: restored, it is tov again without being asked.
+      File.write!(Path.join([root, "torah", "index.json"]), "INDEX")
+      assert eventually(fn -> L1Integrity.status(:tablet38_periodic) == :ok end)
+    end
+
+    test "answers from the last reading while a periodic re-reading is under way" do
+      root = small_scroll()
+      {:ok, pinned} = L1Reader.compute_tree_hash(root)
+
+      {:ok, pid} =
+        L1Integrity.start_link(name: :tablet38_busy, l1_root: root, expected_hash: pinned, interval: :never)
+
+      on_exit(fn -> if Process.alive?(pid), do: GenServer.stop(pid) end)
+
+      # Trigger a periodic reading by hand; the process must not be busy
+      # with it — a status call is answered at once, from the last reading.
+      send(pid, :read)
+      {micros, status} = :timer.tc(fn -> L1Integrity.status(:tablet38_busy) end)
+      assert status == :ok
+      assert micros < 50_000
     end
   end