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/engine/legal.ts

@@ -17,9 +17,38 @@ import { enhancementRule } from './content.ts';
 import { check, areaOf, destinationsFor } from './apply.ts';
 import type { Intent } from './intents.ts';
 import type { GameState, GridCoord, PlayerIndex } from './state.ts';
-import { seatOf } from './state.ts';
+import { coordKey, seatOf } from './state.ts';
+import type { MoveDestination } from './track.ts';
 import { variantsFor } from './track.ts';
 
+/**
+ * A square GridCoord this route passes through that no OTHER route to the same destination does —
+ * the discriminator `switch.move.via` needs so `selectDestination` (apply.ts) never has to guess
+ * which of two routes an intent meant. `siblings` is every route the same `destinationsFor` call
+ * returned at this same square; `route` must be one of them.
+ *
+ * Absent when `route` is the only route there (the overwhelmingly common case — `via` need not be
+ * carried at all), or when the destination is one hop away, which is never ambiguous: the square
+ * immediately before a given entry port is fixed by geometry, so a zero-length path cannot have a
+ * sibling.
+ */
+function distinguishingVia(route: MoveDestination, siblings: readonly MoveDestination[]): GridCoord | undefined {
+  if (route.path.length === 0) return undefined;
+  const otherSquares = new Set<string>();
+  for (const sibling of siblings) {
+    if (sibling === route) continue;
+    for (const step of sibling.path) otherSquares.add(coordKey(step.coord));
+  }
+  for (const step of route.path) {
+    if (!otherSquares.has(coordKey(step.coord))) return step.coord;
+  }
+  // Every square on this route is shared with some sibling — geometrically possible only if the
+  // routes fork and rejoin more than once, which `track.ts`'s doc comment says not to rely on not
+  // happening. Naming the last square before the destination at least matches today's behaviour:
+  // wrong in the same way a route with no `via` at all would be.
+  return route.path[route.path.length - 1]!.coord;
+}
+
 const CAR_TYPES: readonly CarType[] = ['coach', 'boxcar', 'reefer', 'hopper', 'tank', 'caboose'];
 
 /** Every intent `player` may legally submit right now. */
@@ -86,8 +115,20 @@ function localOpsCandidates(s: GameState, player: PlayerIndex): Intent[] {
     if (tray.position.at !== 'grid' || tray.position.seat !== seatOf(s, player)) continue;
     const from = tray.position.coord;
     for (const reverse of [false, true]) {
-      for (const d of destinationsFor(s, player, trayId, from, reverse)) {
-        out.push({ type: 'switch.move', trayId, to: d.coord, reverse });
+      const dests = destinationsFor(s, player, trayId, from, reverse);
+      // Grouped by destination square so `distinguishingVia` only ever compares routes that are
+      // actually racing for the same button — two routes to DIFFERENT squares need no `via` to
+      // tell apart, `to` already does that.
+      const byCoord = new Map<string, MoveDestination[]>();
+      for (const d of dests) {
+        const k = coordKey(d.coord);
+        (byCoord.get(k) ?? byCoord.set(k, []).get(k)!).push(d);
+      }
+      for (const group of byCoord.values()) {
+        for (const d of group) {
+          const via = group.length > 1 ? distinguishingVia(d, group) : undefined;
+          out.push({ type: 'switch.move', trayId, to: d.coord, reverse, ...(via ? { via } : {}) });
+        }
       }
     }
     for (let n = 1; n <= tray.consist.length; n++) {