deps: the pins advance to the reviewed libavr and pureboot, and the image is smaller

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/CMakeLists.txt

@@ -2,7 +2,7 @@ libavr_format_test()
 
 # The image must stay below the boot section at 0x7e00, and the README states
 # what it measures; a library advance that moves it says nothing on its own.
-libavr_size_claim_test(fantemp 7838)
+libavr_size_claim_test(fantemp 7836)
 
 # The README says the two modes emit the same image, and only a tree with
 # both built can say whether they do.