fb/: untrack and gitignore -- throwaway local verification images

Reversing the earlier decision to track fb/ in git (made when first
setting up the directory for item 4.3.2). Captain Bob: these are
disposable screenshots for eyeballing framebuffer output during Console
work, never meant to be committed. Removed from git tracking (git rm
--cached) and added to .gitignore; files that still exist locally are
untouched, deletions already made locally are left as-is.

Also fixes item 4.3.4's amd64-only blind spot found in the process:
aarch64/riscv64 had no framebuffer device at all (GOP: protocol not
found) -- the "all three architectures boot clean" checks run all session
were REPL/dict_hash parity, a different thing from GOP presence, and
conflating the two was an error. Added -device ramfb (EDK2's
firmware-only GOP framebuffer) to both architectures' qemu targets in
Makefile.starkernel. Both now report GOP: linear framebuffer found at
800x600; cube rendering verified correct on both (screenshots not
committed, per the untrack above -- verified visually this session).
Standard three-arch acceptance boot re-run afterward, all clean,
dict_hash identical and unchanged from before this fix.

.gitignore

@@ -34,6 +34,10 @@ experiments/**/results/*.log
 Thumbs.db
 qemu.log
 
+# Framebuffer/Console screendumps — throwaway local verification images,
+# never meant to be committed (Captain Bob, 2026-08-07)
+fb/
+
 # Serial acceptance logs and session directories are audit artifacts — tracked in git
 # logs/ and logs2/ are intentionally NOT ignored
 # EXCEPTION: ECW-trace sessions generate 500-638MB logs that exceed GitHub's 100MB