switching paths: two routes to the same square

Builds docs/plans/switching-paths.md. A passing loop can offer two legal
routes between the same two squares, coupling different cars — the engine
only ever found one, an artifact of search order (Reported by Jesse, undo
379). exploreMoves now enumerates every simple route (per-path visited set,
capped at 4000 frontier nodes) and dedupes on outcome — destination, entry
side, and origin-tagged cars — rather than on reaching the square at all.

switch.move gains an optional `via: GridCoord` naming one intermediate
square on the chosen route; absent, it resolves exactly as before, so
every existing save and bot decision replays identically (575/575, then
579/579 with the new tests). Threaded through the label, the action-list
dedupe, the hover highlight (data-route), and the history (trayMoved.via).

Ruling recorded as Gap 14 in docs/rules/open-questions.md: the player may
choose the path; §A.4's "may not go around" a car does not reach a
different track the player declined to enter.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAt2YCXgd5qCjBcF2x34aK

src/web/main.ts

@@ -656,8 +656,16 @@ function renderDistrict(f: Frame): void {
  * `null` for the actions that act on no square at all — ending a turn, drawing a card, a train card
  * that goes to the timetable — so those buttons carry nothing and highlight nothing.
  */
-function cellRef(coord: { row: number; col: number } | null | undefined): string {
-  return coord ? ` data-square="${coord.row},${coord.col}"` : '';
+function cellRef(
+  coord: { row: number; col: number } | null | undefined,
+  route?: { row: number; col: number }[],
+): string {
+  const square = coord ? ` data-square="${coord.row},${coord.col}"` : '';
+  // Only a `switch.move` with more than one legal route carries this (docs/plans/switching-paths.md)
+  // — every intermediate square the chosen route runs over, so hovering lights the whole road, not
+  // just where it ends.
+  const path = route && route.length > 0 ? ` data-route="${route.map((c) => `${c.row},${c.col}`).join(' ')}"` : '';
+  return square + path;
 }
 
 /**
@@ -676,13 +684,13 @@ function cellRef(coord: { row: number; col: number } | null | undefined): string
  * `data-ghost` target, which is precisely the case where the player is choosing between coordinates.
  * Matching only one of the two would leave the most mistake-prone moment unhelped.
  */
-function wirePointing(node: HTMLElement, key: string): void {
+function wirePointing(node: HTMLElement, key: string, routeKeys: readonly string[] = []): void {
   const grid = $('grid');
+  const keys = [key, ...routeKeys];
   const marks = (): Element[] =>
-    [
-      grid.querySelector(`g[data-cell="${key}"]`),
-      grid.querySelector(`g[data-ghost="${key}"]`),
-    ].filter((g): g is Element => g !== null);
+    keys
+      .flatMap((k) => [grid.querySelector(`g[data-cell="${k}"]`), grid.querySelector(`g[data-ghost="${k}"]`)])
+      .filter((g): g is Element => g !== null);
   const on = (): void => {
     for (const g of marks()) g.classList.add('bs-point');
   };
@@ -744,6 +752,7 @@ function renderActions(
     label: string;
     tip?: string;
     coord?: { row: number; col: number };
+    route?: { row: number; col: number }[];
   }): string => {
     const { label, index } = a;
     const cut = label.indexOf(' — ');
@@ -751,7 +760,7 @@ function renderActions(
     // The menu resolved the card's description server-side, so the page never needs the state.
     const rest = cut > 0 ? label.slice(cut + 3) : (a.tip ?? '');
     return (
-      `<button class="act" data-i="${index}"${cellRef(a.coord)}${rest ? ` data-tip="${esc(rest)}"` : ''}>${esc(head)}</button>`
+      `<button class="act" data-i="${index}"${cellRef(a.coord, a.route)}${rest ? ` data-tip="${esc(rest)}"` : ''}>${esc(head)}</button>`
     );
   };
 
@@ -933,7 +942,8 @@ function renderActions(
         }
       : () => apply(Number(node.dataset['i']));
     const square = node.dataset['square'];
-    if (square) wirePointing(node, square);
+    const route = node.dataset['route'];
+    if (square) wirePointing(node, square, route ? route.split(' ') : []);
   }
 }