docs(roadmap): reopen background-only transparency, fix floating-mode-perf drift

#131/#168 unblock #101 (Background-only transparency, per-pixel alpha) --
wgpu's Dx12SwapchainKind::DxgiFromVisual plus win_opacity::set_no_redirection_bitmap
gives selective frosted-glass transparency without custom DComp integration.
Reopened via the roadmap sync script and re-planned for the main window
(Normal/Always-on-Top/Always-Behind), with dialogs staying opaque.

Also fixes unrelated pre-existing drift: #104 (floating-mode-perf) was closed
manually after architectural review, not completed, but $features still said
kind="planned" -- flipped to "dropped" so the sync script stops erroring.