Heal fullscreen companion chrome on arrival

Display sleep purges the backing stores of windows on non-visible
Spaces. The instant snap arrival never makes the WindowServer re-request
their contents, so a fullscreen app's toolbar companion composites as
nothing until Mission Control forces a repaint; WebKit content
repopulates itself, and the toolbar window exposes no AX element the
repaint nudge could reach (#63).

After every verified fullscreen arrival the navigator now posts a dock
swipe that ramps to ~2pt of slide and cancels: enough of the Dock's
transition choreography that the WindowServer re-requests window
contents, without committing a switch and without visible motion. The
cancelled phase is required - the Dock latches a commit during the
changed ramp and honors it on ended even at zero final progress. The
purge is invisible to every bookkeeping signal short of capturing
pixels and the gesture is imperceptible, so the heal runs
unconditionally.