Draw the board as track, split the site into three pages, and fix curve geometry

Adds SVG board rendering shared by the game and the replay, a splash page with a
shareable replay directory, hover tooltips for reference detail, and makes curves
two-port arcs so sidings and run-arounds can finally be built.

src/engine/apply.ts

@@ -1326,6 +1326,7 @@ function protoCard(
         kind: 'track',
         geometry,
         ...(v.axis ? { axis: v.axis } : {}),
+        ...(v.arc ? { arc: v.arc } : {}),
         ...(v.turnout ? { turnout: v.turnout } : {}),
         ...(v.bypass ? { bypass: v.bypass } : {}),
       },
@@ -1504,6 +1505,7 @@ function protoTrackCard(
       kind: 'track',
       geometry,
       ...(v.axis ? { axis: v.axis } : {}),
+      ...(v.arc ? { arc: v.arc } : {}),
       ...(v.turnout ? { turnout: v.turnout } : {}),
       ...(hand !== 'none' ? { hand } : {}),
     },