fix: make the submit gate a single atomic

9c5ce5f established "a node never sleeps with a wake outstanding" and
implemented it as two independent atomics: queued_ for "a firing is in
flight", wake_pending_ for "a wake arrived during one". Two variables cannot
express that invariant, because the release side has to read and write both
and a wake can land in between:

  producer (try_submit)              worker (release_and_recheck)
  ------------------------           ----------------------------
  CAS reads queued_ == true, fails
                                     queued_.store(false)
                                     wake_pending_.exchange(false) -> false
  wake_pending_.store(true)

queued_ false, wake_pending_ true, nothing running and nothing scheduled —
exactly the state the invariant forbids. This is not a memory-ordering
subtlety; the interleaving holds under seq_cst.

SubmitGate replaces both with one atomic over three states, so "idle" and
"wake outstanding" are the same variable and no interleaving can produce
both. A release that finds a recorded wake keeps the claim and hands it to
the next firing, so the node is never momentarily idle while a submission
for it is in flight.

What this does not do is fix a reproducible hang. Every current call site
follows release_and_recheck() with a level re-check — on_input_ready(), or
outputs_have_space() on the parked path — which rediscovers the state a lost
wake would have signalled. The bug is masked, and I could not write a
node-level test that fails before and passes after; claiming otherwise would
be dishonest. The masking is a property of the call sites, not the
mechanism: any future early return that forgets its re-check reintroduces a
silent hang, and the pipeline has already been round that loop twice
(28e0667, then 9c5ce5f, each of which moved the stall rather than removing
it).

So the tests are structural. The state machine is pinned by contract tests,
and the defect it replaces is pinned by demonstration: LegacyGate in the
test file is the old protocol with a seam between the failed CAS and the
wake record, which makes the loss deterministic rather than something to
wait for. It also keeps the defect on record now that the code implementing
it is gone.

Also ignores build-*/ so a sanitizer build tree cannot be committed by
accident, which this commit did on its first attempt.

tests/CMakeLists.txt

@@ -36,6 +36,7 @@ add_executable(kpn_tests
     test_pool_node.cpp
     test_backpressure_deadlock.cpp
     test_scheduler.cpp
+    test_submit_gate.cpp
 )
 
 target_link_libraries(kpn_tests PRIVATE