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>

session/frametop.conf.example

@@ -7,6 +7,8 @@ WIDTH=1920         # gamescope: pixels per screen (at most 1920x1080 worth)
 HEIGHT=1080
 PHYS_WIDTH=1.6     # gamescope: panel width in metres, docked
 REMOTE=0           # 1 = serve the desktop over VNC on the tailnet (port 5900; see README)
+FLOAT_SLOTS=8      # screens backend: how many windows can float in VR at once (spare outputs; 0 turns it off; restart the desktop after a change)
+FLOAT_MARGIN=300   # floating windows: pixels around each window on its output, so menus have room past its edges
 POINTER=0          # 1 = the mouse drives the universal 3D pointer (ft_pointer driver) instead of a plain mouse
 POINTER_SENSITIVITY=0.03   # degrees per mouse count
 POINTER_IDLE=30    # seconds without mouse use before the controllers get their laser back