Document CI API availability and historical mincrypt layouts

tests/mincrypt-layout-tests.cpp

@@ -21,4 +21,5 @@ C_ASSERT(FIELD_OFFSET(MINCRYPT_CHAIN_ELEMENT, Certificate) == 104);
 #if defined(_M_AMD64)
 // These opt-in declarations establish observations only for the named x64 build.
 #include "Domito.MinCrypt.26100.h"
+#include "Domito.MinCrypt.Historical.h"
 #endif