v0.8.0.9 — the bot plans its switching turn, stops wasting its draws, and the engine walks each route once

The developer bot, re-measured decision by decision against the bot before it,
goes from about -0.3 revenue a game to about 4.8:

- plans the whole switching turn before its first Move (sim/switch-planner.ts),
  +2.89 over 1600 paired seeds; closes TODO #53
- takes a face-up card only if it could play it, +1.52 over 1600 seeds
- stops running Second Sections by accident in the New Train phase, +0.32
- lays track by what the district can do afterwards, +0.12 over 6400 seeds,
  run-arounds in 22 of 60 districts against 9

The engine is 2.8x faster with play proven identical: a route cache scoped to
one unchanged position, applyIntent split into prepareIntent + commitEvents,
and less allocation in exploreMoves. npm test now leaves out the bot
simulations, which run as npm run test:sim.

No rule changed; games in progress resume. Rejected candidates and the
Second Section card question are in CHANGELOG.md and TODO.md (#104-#106).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017nnuCv8UodHucFfx3LWEoX

src/engine/apply.ts

@@ -1508,17 +1508,48 @@ export function selectDestination(
   return chosen ?? atTo[0];
 }
 
+/**
+ * ROUTES, WALKED ONCE PER POSITION.
+ *
+ * A route walk (`reachableDestinations`) was a third of all simulation time, and most of it was the
+ * same walk repeated: `legal.ts` walks a tray's routes to list its moves, then `check` walks them again
+ * for every one of those moves, and `applyIntent` walks the chosen one a third time in `execute`.
+ * Profiled 2026-09-14 with inlining off: `reachableDestinations` 34% inclusive, garbage collection 34%.
+ *
+ * So while one position is being examined — a legal-action listing, or the check and execute of one
+ * intent — a walk is kept and reused. Both scopes read the state and never write it, the key names
+ * everything the walk depends on besides that state, and the cache is keyed to the state OBJECT and
+ * cleared when the scope ends, so a hit returns exactly what a fresh walk would have. Nothing may
+ * mutate a returned route; nothing does.
+ */
+let routeCache: { state: GameState; routes: Map<string, MoveDestination[]> } | null = null;
+
+export function withRouteCache<T>(s: GameState, fn: () => T): T {
+  if (routeCache) return fn();
+  routeCache = { state: s, routes: new Map() };
+  try {
+    return fn();
+  } finally {
+    routeCache = null;
+  }
+}
+
 function destinationsFor(
   s: GameState,
   player: PlayerIndex,
   trayId: TrayId,
   from: GridCoord,
   reverse: boolean,
-) {
+): MoveDestination[] {
+  const cache = routeCache?.state === s ? routeCache.routes : null;
+  const key = cache ? `${player}|${trayId}|${from.row},${from.col}|${reverse ? 1 : 0}` : '';
+  const hit = cache?.get(key);
+  if (hit) return hit;
+
   const tray = s.trays.get(trayId)!;
   const facing = facingPort(s, trayId);
   const exit: Port = reverse ? reversePort(s, player, from, facing) : facing;
-  return reachableDestinations(
+  const routes = reachableDestinations(
     {
       area: areaOf(s, player),
       occupancy: occupancyFor(s, player, trayId),
@@ -1528,6 +1559,8 @@ function destinationsFor(
     from,
     exit,
   );
+  cache?.set(key, routes);
+  return routes;
 }
 
 /**
@@ -2702,7 +2735,8 @@ export function reduce(s: GameState, e: GameEvent): void {
  * index is out of range, which `check` reports rather than silently defaulting — a wrong
  * orientation is a different card, not a detail.
  */
-function protoCard(
+/** Exported for the same reason as `extendLimitsIfNeeded`: the bot builds the card a lay would place exactly as the reducer does. */
+export function protoCard(
   kind: { kind: string; geometry?: string; facility?: string; hand?: string },
   variant: number | undefined,
 ): TrackCard | null {
@@ -3093,7 +3127,8 @@ function applyModifier(area: OfficeArea, coord: GridCoord, modifier: ModifierKin
  * §8.1 and §10 both reason about "the track between the train and the Limits", Interlocking holds
  * an arrival AT the Limits, and running past a player's Limits is what makes a collision his fault.
  */
-function extendLimitsIfNeeded(area: OfficeArea, placed: GridCoord): void {
+/** Exported so the bot can score a lay on a copy of the district by the engine's own rule, not a copy of it. */
+export function extendLimitsIfNeeded(area: OfficeArea, placed: GridCoord): void {
   if (placed.row !== area.runningRow) return;
 
   if (placed.col <= area.limitsWest.col) {
@@ -3287,16 +3322,35 @@ function limitsCard(): TrackCard {
 // Public entry point
 // ---------------------------------------------------------------------------
 
-export function applyIntent(s: GameState, player: PlayerIndex, i: Intent): ApplyResult {
-  const code = check(s, player, i);
-  if (code) return { ok: false, code, message: `${i.type} rejected: ${code}` };
+/**
+ * THE FIRST HALF OF `applyIntent`: decide, without changing anything.
+ *
+ * `check` and `execute` read the same unchanged position, so its routes are walked once between them
+ * (`withRouteCache`). Never writes `s`. Split out for a caller that decides many intents against ONE
+ * position and applies each to a COPY of it — the switching planner — which can then share that
+ * position's routes across every candidate instead of re-walking them on each copy.
+ */
+export function prepareIntent(s: GameState, player: PlayerIndex, i: Intent): ApplyResult {
+  const prepared = withRouteCache(s, (): { code: RejectionCode } | { events: GameEvent[] } => {
+    const code = check(s, player, i);
+    return code ? { code } : { events: execute(s, player, i) };
+  });
+  if ('code' in prepared) return { ok: false, code: prepared.code, message: `${i.type} rejected: ${prepared.code}` };
+  return { ok: true, events: prepared.events };
+}
 
-  const events = execute(s, player, i);
+/** THE SECOND HALF: fold events `prepareIntent` produced into a state equal to the one it read. */
+export function commitEvents(s: GameState, events: readonly GameEvent[]): void {
   for (const e of events) reduce(s, e);
   // Gitea#16 — the intent half of the fold; `advance` does the phase driver's half. See `tally.ts`
   // for why it cannot simply live inside `reduce`.
   for (const e of events) tallyEvent(s, e);
-  return { ok: true, events };
+}
+
+export function applyIntent(s: GameState, player: PlayerIndex, i: Intent): ApplyResult {
+  const r = prepareIntent(s, player, i);
+  if (r.ok) commitEvents(s, r.events);
+  return r;
 }
 
 export { isOperationalRail, destinationsFor };