feat: add Phase 1 core pathfinding architecture

Implements the new Baritone-inspired A* pathfinding system:
- Core types: PathNode, PathResult, MoveResult, MoveType, ActionCosts
- BinaryHeapOpenSet min-heap for A* open set
- AStarPathFinder with timeout, cancellation, partial path support
- CalculationContext for thread-safe world state snapshots
- MoveHelper for block passability checks
- IGoal interface + GoalBlock, GoalXZ, GoalNear, GoalComposite
- IMove interface + MoveTraverse, MoveDiagonal, MoveAscend, MoveDescend, MoveClimb
- /pathfind command for testing the new pathfinder

Made-with: Cursor

MinecraftClient/Pathing/Core/MoveResult.cs

@@ -0,0 +1,28 @@
+namespace MinecraftClient.Pathing.Core
+{
+    /// <summary>
+    /// Result of an IMove.Calculate() call. Mutable struct passed by ref for zero-alloc hot path.
+    /// </summary>
+    public struct MoveResult
+    {
+        public int DestX;
+        public int DestY;
+        public int DestZ;
+        public double Cost;
+
+        public void Set(int x, int y, int z, double cost)
+        {
+            DestX = x;
+            DestY = y;
+            DestZ = z;
+            Cost = cost;
+        }
+
+        public void SetImpossible()
+        {
+            Cost = ActionCosts.CostInf;
+        }
+
+        public readonly bool IsImpossible => Cost >= ActionCosts.CostInf;
+    }
+}