docs(roadmap): document #169 floating-mode DComp investigation, blocked

Applying WS_EX_NOREDIRECTIONBITMAP to a floating panel's HWND (one of N
separate show_viewport_immediate OS windows) renders it as a washed-out
light gray/blue instead of translucent, reproduced with zero opacity logic
in the drawn content -- isolated via controlled tests to rule out premultiply
bugs or a WS_EX_LAYERED conflict. Points to a real eframe/egui-wgpu
limitation in secondary-viewport surface creation combined with a DComp
backend, not a mistake in the #101 approach. Reverted all floating-mode code
changes; floating mode keeps its existing WS_EX_LAYERED uniform-dim opacity,
unaffected and unregressed.