fix(data): level-per-emission pyramid cover, relaxed-atomic ring slots

cover_buckets chose one level per pass, so an odd first whole bucket (half of all

measurement ranges) never reached a level above 0 and folded every remaining

bucket at level 0 -- 15624 entries for a 1e6-sample pyramid. Pick the largest

aligned, in-range, materialised level at each cursor instead; the new cost test

counts folded entries through a PLOT_PYRAMID_COUNT_COVERS-only hook (absent from

normal builds) and fails on the old body.

Ring slots are now relaxed std::atomic: overwrite-on-full means the producer

reuses the slot of a snapshot's oldest sample while the consumer reads it, which

was a data race, not just a stale value. copy_from loads explicitly instead of

std::copy; push stays lock-free. A new producer/consumer case is the one TSan

flags without this.

Also extends the wrapped-window test with x0/bounds/pyramid/sum/sumsq/gather

checks, and corrects the StreamSeries threading note to the sample-count bound.

CMakeLists.txt

@@ -22,6 +22,11 @@ target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})
 plot_apply_warnings(plot_core)
 
 if(PLOT_BUILD_TESTS)
+  # Test-only: counts the pyramid's covering-decomposition entries so a test can
+  # pin its O(log n) cost, which no query answer reveals. Off in every non-test
+  # build (see include/plot/data/detail/cover_count.hpp).
+  target_compile_definitions(plot_core PRIVATE PLOT_PYRAMID_COUNT_COVERS)
+
   enable_testing()
   add_subdirectory(tests)
 endif()