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.

Makefile.starkernel

@@ -816,6 +816,7 @@ else ifeq ($(ARCH),aarch64)
 		-drive if=virtio,format=raw,file=$(QEMU_ISO),media=cdrom \
 		-drive id=artdisk,file=$(ARTDISK),format=raw,if=none \
 		-device virtio-blk-pci,drive=artdisk \
+		-device ramfb \
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \
 		-display none \
@@ -915,6 +916,7 @@ else ifeq ($(ARCH),riscv64)
 		-device virtio-blk-pci,drive=hd0,addr=0x2 \
 		-drive id=artdisk,file=$(ARTDISK),format=raw,if=none \
 		-device virtio-blk-pci,drive=artdisk,addr=0x1 \
+		-device ramfb \
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \
 		-display none \