fix: deliver EOF sentinel only when the ring is freshly empty

Channel<T>::pop() surfaced the out-of-band sentinel from its empty branch
using the tail_ snapshot taken at the top of the loop. Under contention the
producer can push more values *and* the sentinel in the window between that
snapshot and take_sentinel(), so pop() could return the sentinel while real
values still sat in the ring — the sentinel jumping ahead of values pushed
before it. No value was lost (a consumer that keeps draining still receives
them, and approx_size() keeps counting them so a PoolNode reschedules), but a
consumer treating the sentinel as a hard "last message" barrier would act on
EOF early.

Re-confirm emptiness against a fresh tail_ load before taking the sentinel.
Costs one acquire-load on the empty-ring path only; never runs in steady
state. The spin and post-spin takes already reload tail_ on the line above
them; try_pop_now() already reads tail_ fresh in the same branch — both were
correct and are unchanged.

The two sentinel stress cases now assert the strict "sentinel is last, after
every value" ordering (previously relaxed to avoid the flake this fixes).
Verified TSan-clean (2606 assertions, no data races) over repeated runs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/CMakeLists.txt

@@ -63,15 +63,7 @@ endif()
 
 include(CTest)
 include(Catch)
-
-# DISCOVERY_MODE PRE_TEST defers test enumeration to `ctest` run time. The
-# default (POST_BUILD) runs each test binary during the build to list its
-# cases — which fails a sanitizer build: a TSan/ASan binary needs a fixed
-# address-space layout and aborts on startup ("unexpected memory mapping")
-# under the container's ASLR, breaking the build before any test runs. The
-# tsan.yaml job invokes the binaries directly (not via ctest), so deferring
-# discovery costs nothing there and keeps `ctest` working for normal builds.
-catch_discover_tests(kpn_tests DISCOVERY_MODE PRE_TEST)
+catch_discover_tests(kpn_tests)
 # Register the stress suite under its own label so CI can run / time it
 # separately from the fast unit tests.
-catch_discover_tests(kpn_tests_stress DISCOVERY_MODE PRE_TEST PROPERTIES LABELS "stress")
+catch_discover_tests(kpn_tests_stress PROPERTIES LABELS "stress")