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.

cmake/plot_umbrella_check.cmake

@@ -0,0 +1,76 @@
+# Keeps include/plot/plot.hpp honest against the plan's Global Constraints:
+# every public header under include/plot/ must be included by the umbrella, and
+# every <plot/...> the umbrella names must be a file that actually exists.
+#
+# "Public" is everything the recursive glob finds except the macro-gated
+# */detail/ instrumentation, which is documented as not part of the surface and
+# deliberately stays out of the umbrella.
+#
+# Runs twice: tests/CMakeLists.txt includes it at configure time (so a header
+# added without its umbrella line stops the configure rather than the run), and
+# ctest runs it again as `plot_umbrella_covers_public_headers` via `cmake -P`.
+# Both read the same file so a header the build sees is a header this checks --
+# there is no hand-maintained list to fall out of date.
+#
+# Inputs (set by the including scope, or passed with -D in script mode):
+#   PLOT_INCLUDE_DIR  the installed-header root, e.g. <source>/include
+#   PLOT_UMBRELLA     the umbrella header, e.g. <PLOT_INCLUDE_DIR>/plot/plot.hpp
+
+if(NOT DEFINED PLOT_INCLUDE_DIR)
+  message(FATAL_ERROR "plot: PLOT_INCLUDE_DIR is not set")
+endif()
+if(NOT DEFINED PLOT_UMBRELLA)
+  set(PLOT_UMBRELLA "${PLOT_INCLUDE_DIR}/plot/plot.hpp")
+endif()
+
+file(GLOB_RECURSE plot_umbrella_headers
+  RELATIVE "${PLOT_INCLUDE_DIR}"
+  "${PLOT_INCLUDE_DIR}/plot/*.hpp")
+list(SORT plot_umbrella_headers)
+file(READ "${PLOT_UMBRELLA}" plot_umbrella_text)
+
+# The umbrella is a public header too, but it cannot include itself.
+file(RELATIVE_PATH plot_umbrella_relative "${PLOT_INCLUDE_DIR}" "${PLOT_UMBRELLA}")
+
+# Every other public header is named by the umbrella.
+set(plot_umbrella_missing "")
+foreach(header IN LISTS plot_umbrella_headers)
+  if(header MATCHES "/detail/" OR header STREQUAL plot_umbrella_relative)
+    continue()
+  endif()
+  string(FIND "${plot_umbrella_text}" "#include <${header}>" plot_umbrella_pos)
+  if(plot_umbrella_pos EQUAL -1)
+    list(APPEND plot_umbrella_missing "${header}")
+  endif()
+endforeach()
+
+if(plot_umbrella_missing)
+  string(REPLACE ";" "\n  " plot_umbrella_missing_text "${plot_umbrella_missing}")
+  message(FATAL_ERROR
+    "plot: ${PLOT_UMBRELLA} does not include every public header under "
+    "include/plot/ -- missing:\n  ${plot_umbrella_missing_text}\n"
+    "Add each as `#include <plot/...>`, or move it under a */detail/ directory "
+    "if it is implementation-only and not part of the public surface.")
+endif()
+
+# And every header the umbrella names is a file that exists (a rename or a
+# deleted header cannot leave a dangling include behind).
+string(REGEX MATCHALL "#include <plot/[^>]+>" plot_umbrella_includes "${plot_umbrella_text}")
+set(plot_umbrella_dangling "")
+foreach(plot_umbrella_include IN LISTS plot_umbrella_includes)
+  string(REGEX REPLACE "#include <([^>]+)>" "\\1" plot_umbrella_header "${plot_umbrella_include}")
+  if(NOT EXISTS "${PLOT_INCLUDE_DIR}/${plot_umbrella_header}")
+    list(APPEND plot_umbrella_dangling "${plot_umbrella_header}")
+  endif()
+endforeach()
+
+if(plot_umbrella_dangling)
+  string(REPLACE ";" "\n  " plot_umbrella_dangling_text "${plot_umbrella_dangling}")
+  message(FATAL_ERROR
+    "plot: ${PLOT_UMBRELLA} includes headers that do not exist under "
+    "include/plot/:\n  ${plot_umbrella_dangling_text}")
+endif()
+
+list(LENGTH plot_umbrella_headers plot_umbrella_header_count)
+message(STATUS
+  "plot: umbrella covers every public header (${plot_umbrella_header_count} headers globbed)")