Extract scanout from the compositor into backend.zig — a `Backend` tagged union
with info()/surface()/present(damage) and canModeSet/hasVsync flags. The v1 GOP
path becomes `backend.Gop` (claim the display node, WC-map the LFB, keep the
cacheable back buffer, present = the damage-rect WC copy); display.zig now
composes into backend.surface() and calls backend.present(damage), with no LFB or
framebuffer geometry left in the compositor core. The selection decision is the
pure chooseKind(native_available), split from the syscall-bound bring-up, ready
for the native-if-present branch at V4.
Pure refactor: display-service, display-demo, and zig build test all pass
unchanged.
Daniel Samson