fb/: split into per-ISA subdirectories (amd64/aarch64/riscv64)

Screenshots stay tracked in git, same as before -- just organized so
verification images are easy to find per architecture as Console work
progresses across all three ISAs. scripts/qemu_screenshot.sh (amd64-only)
now writes into fb/amd64/.

scripts/qemu_screenshot.sh

@@ -11,7 +11,8 @@
 # Outputs:
 #   <BUILD_DIR>/screenshot.ppm                 — raw PPM from QEMU screendump
 #   <LOG_DIR>/qemu-screenshot-YYYYMMDD-HHMMSS.log  — serial log
-#   fb/qemu-screenshot-YYYYMMDD-HHMMSS.png         — PNG (tracked in git, not build/)
+#   fb/amd64/qemu-screenshot-YYYYMMDD-HHMMSS.png   — PNG (tracked in git, not build/)
+#   (amd64-only script; fb/ has one subdirectory per ISA by convention)
 #
 # Requires: qemu-system-x86_64, socat, xorriso, mtools, ovmf, imagemagick
 
@@ -62,7 +63,7 @@ fi
 # --------------------------------------------------------------------------
 # Setup paths
 # --------------------------------------------------------------------------
-FB_DIR="fb"
+FB_DIR="fb/amd64"
 mkdir -p "${LOG_DIR}" "${BUILD_DIR}" "${FB_DIR}"
 
 TS=$(date +%Y%m%d-%H%M%S)