test(coverage): a reproducible coverage preset, report and floor check

CMakeLists.txt

@@ -123,6 +123,24 @@ if(PLOT_BUILD_TESTS)
   add_subdirectory(tests)
 endif()
 
+# The reproducible coverage report. A custom target rather than a ctest case,
+# for two reasons: a ctest entry registered in the coverage preset would either
+# fail in a freshly configured tree (no report exists until tools/coverage.sh
+# has run once, which would break the plain `ctest --test-dir build/cov` the
+# preset's own documentation promises) or it would have to build the library
+# and re-run the whole suite inside the test, duplicating the script. A target
+# that is not part of `all` costs the ordinary presets nothing -- and it is not
+# even created there, because PLOT_COVERAGE is off and the option's guard
+# rejects a tree that carries no --coverage flags. The exact command a reader
+# gets is `cmake --build build/cov --target coverage`, which is the same one
+# tools/coverage.sh implements.
+if(PLOT_COVERAGE)
+  add_custom_target(coverage
+    COMMAND "${CMAKE_SOURCE_DIR}/tools/coverage.sh" "${CMAKE_BINARY_DIR}"
+    COMMENT "coverage: building, running the suite, capturing src/** and enforcing the floor"
+    USES_TERMINAL VERBATIM)
+endif()
+
 # Install and export. The core always goes in; each adapter joins the set only if
 # its toolkit can be re-found on a consumer's machine, which the adapter decided
 # when it was added (plot_record_adapter_export() -- the decisions and the reasons