The other half of the same measurement. `wXCoord` and `wYCoord` are the tile the step began on
until the frame it ends, so the stood ledger recorded ground the fly had already left and the tile
under it stayed unstood: `path::frontier` kept offering it, `GO FRONTIER` was dealt aiming one tile
away, and `Arrival::Step` reported `done` the instant the step it did not make landed. A macro that
completes without changing anything, which is section 12.2's trap.
A step that has begun always finishes -- the cartridge owns the animation and no press stops it --
and the screen has already centred on the tile, so it is ground this run has covered. It clears the
map's frontier mark on the same rule the coordinates do: only ground never stood on before.
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