feat(arena): add deterministic headless RL environments

src/interiors/walker.ts

@@ -67,6 +67,8 @@ export interface WalkerController {
   tick(elapsedSeconds: number, action: WalkerAction): WalkerState;
   /** Return to the original spawn, or atomically adopt another valid spawn. */
   reset(spawn?: WalkerSpawn): WalkerState;
+  /** Restore a trusted JSON snapshot without changing the configured spawn. */
+  restore(snapshot: WalkerState): WalkerState;
 }
 
 /**
@@ -138,7 +140,20 @@ export function createWalker(plan: WalkerPlan, options: WalkerOptions): WalkerCo
     return snapshot();
   }
 
-  return { state: snapshot, tick, reset };
+  function restore(next: WalkerState): WalkerState {
+    if (
+      next.levelId !== spawn.levelId || !finitePoint(next.position) || !finitePoint(next.facing) ||
+      !Number.isFinite(next.distance) || next.distance < 0 ||
+      !validPosition(plan, next.levelId, next.position, radius)
+    ) throw new RangeError("walker snapshot is incompatible or invalid");
+    position = copy(next.position);
+    facing = normalizedFacing(next.facing);
+    distance = next.distance;
+    accumulator = 0;
+    return snapshot();
+  }
+
+  return { state: snapshot, tick, reset, restore };
 }
 
 function normalizedFacing(value: Point2 | undefined): Point2 {