fix(build): keep counter instrumentation out of the installed release library

The linux-release preset inherited PLOT_BUILD_TESTS=ON, so the release build
compiled the pyramid cover/merge and ring upload/wait/slot counters into
libplot_core.a and cmake --install shipped them. Set PLOT_BUILD_TESTS=OFF in the
preset; the suite keeps running in linux-debug and linux-asan, which are
unchanged.

Also pin the two header properties the plan calls Global Constraints and that
nothing guarded: every header under include/plot/ compiles standalone, and
include/plot/plot.hpp includes every public one. tests/CMakeLists.txt globs the
headers at configure time (CONFIGURE_DEPENDS) and generates one TU per header
plus an umbrella TU that constructs Plot, Renderer, data::StreamSeries and
input::Controller; a ctest entry builds the header TUs and runs the umbrella,
and cmake/plot_umbrella_check.cmake fails configure and ctest when a public
header is missing from the umbrella.

tests/CMakeLists.txt

@@ -85,3 +85,74 @@ if(PLOT_ASAN_ENABLED)
   set_tests_properties(plot_tests PROPERTIES ENVIRONMENT
     "ASAN_OPTIONS=detect_leaks=1:print_suppressions=1:suppressions=${CMAKE_SOURCE_DIR}/cmake/plot_asan_suppressions.txt")
 endif()
+
+# --- public headers: standalone compilation and umbrella coverage -------------
+#
+# The plan's Global Constraints require every header under include/plot/ to
+# compile standalone, and include/plot/plot.hpp to include every public one
+# (never an adapter). Both were verified by hand and nothing kept them true, so
+# a header added later could break either silently. These targets make the build
+# enforce both and the ctest entries below make the suite fail when the checks
+# do. The header list is globbed at configure time (CONFIGURE_DEPENDS, so adding
+# or removing a header re-globs) -- a hand-kept list would defeat the point.
+file(GLOB_RECURSE plot_public_headers CONFIGURE_DEPENDS
+  RELATIVE "${CMAKE_SOURCE_DIR}/include"
+  "${CMAKE_SOURCE_DIR}/include/plot/*.hpp")
+list(SORT plot_public_headers)
+
+# One generated TU per header, each holding only that header's #include. An
+# OBJECT library: this is a compile check, with no link step, so any header that
+# needs something it does not itself include fails the build here.
+set(plot_header_check_sources "")
+foreach(plot_header IN LISTS plot_public_headers)
+  string(MAKE_C_IDENTIFIER "${plot_header}" plot_header_slug)
+  set(plot_header_check_source "${CMAKE_CURRENT_BINARY_DIR}/header_checks/${plot_header_slug}.cpp")
+  set(plot_header_check_content
+    "// Generated by tests/CMakeLists.txt: <${plot_header}> must compile on its own.\n#include <${plot_header}>\n")
+  file(GENERATE OUTPUT "${plot_header_check_source}" CONTENT "${plot_header_check_content}")
+  list(APPEND plot_header_check_sources "${plot_header_check_source}")
+endforeach()
+
+add_library(plot_header_self_containment OBJECT ${plot_header_check_sources})
+target_link_libraries(plot_header_self_containment PRIVATE plot::core)
+plot_apply_warnings(plot_header_self_containment)
+
+# The umbrella on its own: the generated TU includes only <plot/plot.hpp> and
+# constructs one of each layer's entry type, so an umbrella that misses a header
+# -- or that is unusable without further includes -- fails to build. The test
+# runs the linked binary, so a build that produced nothing usable still fails.
+set(plot_umbrella_check_source "${CMAKE_CURRENT_BINARY_DIR}/umbrella_check.cpp")
+file(GENERATE OUTPUT "${plot_umbrella_check_source}" CONTENT
+"// Generated by tests/CMakeLists.txt: the umbrella must be usable by itself.
+#include <plot/plot.hpp>
+
+int main() {
+  plot::Plot plot;
+  plot::Renderer renderer;
+  plot::data::StreamSeries series(16, 0.0, 1.0);
+  plot::input::Controller controller(plot);
+  (void)renderer;
+  (void)series;
+  (void)controller;
+  return 0;
+}
+")
+add_executable(plot_umbrella_check "${plot_umbrella_check_source}")
+target_link_libraries(plot_umbrella_check PRIVATE plot::core)
+plot_apply_warnings(plot_umbrella_check)
+
+# Coverage check on the umbrella. Included here so a public header missing from
+# plot.hpp stops the configure; the ctest entry below runs the same script again
+# so the check is also a visible test.
+set(PLOT_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/include")
+set(PLOT_UMBRELLA "${CMAKE_SOURCE_DIR}/include/plot/plot.hpp")
+include("${CMAKE_SOURCE_DIR}/cmake/plot_umbrella_check.cmake")
+
+add_test(NAME plot_headers_self_contained
+  COMMAND ${CMAKE_COMMAND} --build "${CMAKE_BINARY_DIR}" --target plot_header_self_containment)
+add_test(NAME plot_umbrella_covers_public_headers
+  COMMAND ${CMAKE_COMMAND}
+          -DPLOT_INCLUDE_DIR=${CMAKE_SOURCE_DIR}/include
+          -DPLOT_UMBRELLA=${CMAKE_SOURCE_DIR}/include/plot/plot.hpp
+          -P "${CMAKE_SOURCE_DIR}/cmake/plot_umbrella_check.cmake")
+add_test(NAME plot_umbrella_check COMMAND plot_umbrella_check)