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.

CMakePresets.json

@@ -23,10 +23,12 @@
     {
       "name": "linux-release",
       "displayName": "Linux Release",
+      "description": "Release artifact for install: PLOT_BUILD_TESTS is OFF so the installed libplot_core.a carries none of the test-only counter instrumentation (PLOT_PYRAMID_COUNT_*, PLOT_RING_STATS). The suite runs in linux-debug and linux-asan.",
       "generator": "Ninja",
       "binaryDir": "${sourceDir}/build/release",
       "cacheVariables": {
         "CMAKE_BUILD_TYPE": "Release",
+        "PLOT_BUILD_TESTS": false,
         "PLOT_BUILD_GLFW": true,
         "PLOT_BUILD_IMGUI": true,
         "PLOT_BUILD_EXAMPLES": true,