Show floating windows as panels of their own

ft-screens makes a panel for each spare output (frametop.float.N), hidden
until ft-floatd floats a window on it. The panel shows only the window's
rectangle of the buffer at the density of the screen it came from, its
popups and dialogs get small panels over it, pressing its title bar
carries it while KWin's pointer stays put, the corner tab resizes the
window in pixels, and two more buttons close it and put it back on the
desktop. The session adds FLOAT_SLOTS spare outputs to KWin and starts
ft-floatd; ft-layout arranges only the screens' outputs, and the pointer
helper treats the new panels like screens. Not yet tried in the headset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

screens/test/headless.sh

@@ -6,7 +6,7 @@
 # or on the Frame. Build ft-screens first (screens/build.sh).
 #
 #   headless.sh start [SCREENS] [SPARES]   (default 2 screens, 1920x1080 and 1080x1920, and
-#                                          3 spare 800x600 outputs, disabled once KWin is up)
+#                                          3 spare outputs, disabled once KWin is up)
 #   headless.sh stop
 #   headless.sh ask '<ft-screens command>'  e.g. toplevels, "input 1 down 400 20"
 #   headless.sh kd <kscreen-doctor args>    e.g. -o, output.WL-2.enable
@@ -89,7 +89,7 @@ case "${1:-}" in
     args=(--screen 1920x1080@1.6 --screen 1080x1920@0.9)
     for ((i = 2; i < screens; i++)); do args+=(--screen 1920x1080@1.6); done
     args=("${args[@]:0:$((screens * 2))}")
-    for ((i = 0; i < spares; i++)); do args+=(--screen 800x600); done
+    args+=(--spares "$spares")
     cd "$REPO_ROOT"
     nohup "$HOME/.local/bin/distrobox" enter "$FRAME_BOX" -- env XDG_RUNTIME_DIR="$rt" \
       ./screens/build/ft-screens --no-vr --socket ft-test-0 --control ft_screens_test "${args[@]}" \