Merge floating-windows into experimental

Floating windows join the keyboard and the hand cutouts. Floating panels
don't get cutouts yet: their panel and popups show crops of the client
buffer (texture bounds), which the side-by-side cutout buffer doesn't
match. KWin gets both the spare outputs and our input method, and the
pointer helper's frametop. prefix already covers the float panels.

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