test: the README's cross-mode claim is now checked, not just stated

The pin advances to the commit that gives consumers the gate's own image
comparison, and the claim written into this README a few commits ago - 8380 B,
byte-identical between the two modes - is registered against it. It compares
clean over every loadable section.

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

test/CMakeLists.txt

@@ -4,6 +4,10 @@ libavr_format_test()
 # what it measures; a library advance that moves it says nothing on its own.
 libavr_size_claim_test(fantemp 8380)
 
+# The README says the two modes emit the same image, and only a tree with
+# both built can say whether they do.
+libavr_mode_identity_test(fantemp)
+
 # The battery is a compile: a static_assert that fails is the failure. It is a
 # target rather than only a ctest so a plain build catches a regression too.
 add_library(consteval_tests OBJECT consteval.cpp)