test: the walk fixture states the board's fuses, not an approximation of them

The profile called the ssd1306 board's own read ff ff ff dc; the part reads
ff ff fd dc - an Atmel-ICE and the loader agree on that byte. Only the high
fuse decides the walk region, so nothing about the refusal changes; the
fixture now names a configuration that was read off hardware.

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

test/pbwalk.py

@@ -50,9 +50,10 @@ def main():
 
     os.makedirs(workdir, exist_ok=True)
     dump = os.path.join(workdir, "dump.bin")
-    # The ssd1306 board's own fuses, and the same boot section with reset left
-    # on the application - the one difference that decides the refusal.
-    bricking, harmless = "ffffffdc", "ffffffdd"
+    # The ssd1306 board's own fuses, as an Atmel-ICE reads them off the part,
+    # and the same boot section with reset left on the application - the one
+    # difference that decides the refusal.
+    bricking, harmless = "fffffddc", "fffffddd"
 
     def session(fuses):
         return pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, fuses=fuses)