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/web/splash.ts

@@ -0,0 +1,9 @@
+/** The splash page. Draws ties on the rule so the line reads as track rather than a border. */
+const ties = document.getElementById('ties');
+if (ties) {
+  let out = '';
+  for (let x = 4; x < 700; x += 9) {
+    out += `<line class="tie" x1="${x}" y1="6" x2="${x}" y2="19"/>`;
+  }
+  ties.innerHTML = out;
+}