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/Goals/GoalBlock.cs

@@ -0,0 +1,42 @@
+using System;
+
+namespace MinecraftClient.Pathing.Goals
+{
+    public sealed class GoalBlock : IGoal
+    {
+        public int X { get; }
+        public int Y { get; }
+        public int Z { get; }
+
+        public GoalBlock(int x, int y, int z)
+        {
+            X = x;
+            Y = y;
+            Z = z;
+        }
+
+        public bool IsInGoal(int x, int y, int z)
+            => x == X && y == Y && z == Z;
+
+        public double Heuristic(int x, int y, int z)
+        {
+            int dx = Math.Abs(x - X);
+            int dy = Math.Abs(y - Y);
+            int dz = Math.Abs(z - Z);
+            return DistanceHeuristic(dx, dy, dz);
+        }
+
+        internal static double DistanceHeuristic(int dx, int dy, int dz)
+        {
+            int horizontal = Math.Max(dx, dz);
+            int diagonal = Math.Min(dx, dz);
+            int straight = horizontal - diagonal;
+            double cost = diagonal * Core.ActionCosts.SprintOneBlock * Core.ActionCosts.DiagonalMultiplier
+                        + straight * Core.ActionCosts.SprintOneBlock
+                        + Math.Abs(dy) * Core.ActionCosts.SprintOneBlock;
+            return cost;
+        }
+
+        public override string ToString() => $"GoalBlock({X}, {Y}, {Z})";
+    }
+}