test: the format and ASCII rules stop being a habit, and the stated sizes stop drifting

libavr's guidance binds this repo too, and until now nothing here checked it -
`ctest` runs `libavr_format_test()` over this tree's own sources now (rules 11
and 33), skipping rather than passing where clang-format is absent. It caught
drift on its first run: a file written this week and edited after formatting.

Where the README states a measured size, `libavr_size_claim_test()` holds it to
the image and holds the image to the prose: advancing the library pin moved
three of these across the fleet with nothing saying so, and re-recording one
now requires the sentence that quotes it to move too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -20,6 +20,8 @@ add_library(ds3231 INTERFACE)
 target_include_directories(ds3231 INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/include)
 target_link_libraries(ds3231 INTERFACE libavr)
 
+include(${LIBAVR_ROOT}/cmake/checks.cmake)
+
 if(PROJECT_IS_TOP_LEVEL)
 	add_subdirectory(example)
 	enable_testing()