default-path gate: mouse mirror catches up to the 720x520 star chart

The waypoint delete/drag assertions failed not because explorer broke but because 2be4473 grew the logical canvas 500x400 -> 720x520 (the Eastern continent) and updated explorer's three coordinate sites while the gate's screen-coord mirror kept dividing by 500/400

▶ Play it: Barometric Stampede — the genre that is mostly drums, so the kit has nowhere to hide https://stellate.app/?genre=jungle&seed=5995&path=1718.20504,1737.20537,1699.20537&xdur=13.122

6b41a82 https://github.com/aboard-io/stellate/commit/6b41a8236197ca3d433b4d6090c1272248670591