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/Moves/Impl/MoveDiagonal.cs

@@ -0,0 +1,54 @@
+using MinecraftClient.Pathing.Core;
+
+namespace MinecraftClient.Pathing.Moves.Impl
+{
+    /// <summary>
+    /// Diagonal walk (1 block in both X and Z, same Y).
+    /// Checks both intermediate cardinal columns for clearance.
+    /// </summary>
+    public sealed class MoveDiagonal : IMove
+    {
+        public MoveType Type => MoveType.Diagonal;
+        public int XOffset { get; }
+        public int ZOffset { get; }
+        public bool DynamicY => false;
+
+        public MoveDiagonal(int xOffset, int zOffset)
+        {
+            XOffset = xOffset;
+            ZOffset = zOffset;
+        }
+
+        public void Calculate(CalculationContext ctx, int x, int y, int z, ref MoveResult result)
+        {
+            int destX = x + XOffset;
+            int destZ = z + ZOffset;
+
+            if (!ctx.CanWalkThrough(destX, y, destZ) || !ctx.CanWalkThrough(destX, y + 1, destZ))
+            {
+                result.SetImpossible();
+                return;
+            }
+
+            if (!ctx.CanWalkOn(destX, y - 1, destZ))
+            {
+                result.SetImpossible();
+                return;
+            }
+
+            if (!ctx.CanWalkThrough(x + XOffset, y, z) || !ctx.CanWalkThrough(x + XOffset, y + 1, z))
+            {
+                result.SetImpossible();
+                return;
+            }
+
+            if (!ctx.CanWalkThrough(x, y, z + ZOffset) || !ctx.CanWalkThrough(x, y + 1, z + ZOffset))
+            {
+                result.SetImpossible();
+                return;
+            }
+
+            result.Set(destX, y, destZ, ctx.SprintCost * ActionCosts.DiagonalMultiplier);
+        }
+    }
+}