test: the image's size stops being a number nobody holds

Every other port in the fleet states its image size and is held to it; this
one stated none, so a library advance could move the firmware and no check
would say a word. 8380 B is what the presets build today, byte-identical
between the two modes, and the README now says so.

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

README.md

@@ -66,6 +66,9 @@ cmake --preset atmega328p-generated
 cmake --build --preset atmega328p-generated
 ```
 
+The firmware is **8380 B** of flash, byte-identical between the generated and
+reflect modes, and `ctest` holds it to that number.
+
 ## Atmel Studio
 
 `master` carries a Studio solution, so this branch does too: `ide/fantemp.atsln`