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/state.ts

@@ -58,6 +58,13 @@ export type TurnoutOrientation = { stem: 'n' | 's' | 'e' | 'w'; through: 'n' | '
  * how many run each way, yet a layout with no north-south straight can never use the Office's
  * junction stubs at all.
  */
+/**
+ * A curve joins two ADJACENT edges. There are exactly four such arcs, and a card can be turned to
+ * any of them — which is what makes a siding possible: an arc reaching NORTH is the only way back
+ * up to the Running Track from the district below.
+ */
+export type TrackArc = 'ne' | 'nw' | 'se' | 'sw';
+
 export type TrackAxis = 'ew' | 'ns';
 
 export type CardGeometry =
@@ -66,6 +73,8 @@ export type CardGeometry =
       geometry: TrackGeometry;
       turnout?: TurnoutOrientation;
       axis?: TrackAxis;
+      /** Which two edges a CURVE joins — chosen on placement, since the card can be turned. */
+      arc?: TrackArc;
       /** Curves are printed left- or right-handed (design supply); not chosen on placement. */
       hand?: 'left' | 'right';
     }