Captures now run inside a dedicated nested Hyprland session by default
(--no-isolate to opt out), so nothing on the operator's live desktop can
leak into a screenshot and the capture never flashes across their screen
either. The nested instance nests as a Wayland client of the outer session
(true headless was ruled out empirically: this machine's real GPU/output is
already claimed by the live session, and only one process can hold logind's
seat at a time), gets floated/exact-resized/focused via one-shot outer-session
hyprctl dispatches targeted by pid, and has Hyprland's default background/
logo and startup warning overlays disabled via config so captures come out
clean. Focusing turned out to be load-bearing, not cosmetic: an occluded
nested window never gets frame callbacks from the outer compositor, so grim
run inside it hangs forever waiting on a ready event that never comes.
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