feat: implement physics engine for player movement and collision detection

- Introduced a comprehensive physics engine that replicates Minecraft's movement mechanics, including player input handling, gravity, and collision detection.
- Added classes for player physics, movement input, and collision detection, ensuring accurate simulation of player interactions with the game world.
- Integrated AABB (Axis-Aligned Bounding Box) structures for precise collision detection against blocks.
- Enhanced movement capabilities with support for jumping, sneaking, and sprinting, along with step-up mechanics for navigating terrain.

These changes significantly improve the realism and responsiveness of player movement within the game environment.

MinecraftClient/Physics/MovementInput.cs

@@ -0,0 +1,55 @@
+using System;
+
+namespace MinecraftClient.Physics
+{
+    /// <summary>
+    /// Represents movement input state, equivalent to vanilla ClientInput / KeyboardInput.
+    /// </summary>
+    public class MovementInput
+    {
+        public bool Forward;
+        public bool Back;
+        public bool Left;
+        public bool Right;
+        public bool Jump;
+        public bool Sneak;
+        public bool Sprint;
+
+        /// <summary>
+        /// Get the raw input vector (xxa, zza) before rotation.
+        /// Forward = +zza, Back = -zza, Left = +xxa, Right = -xxa.
+        /// Then normalized if magnitude > 1.
+        /// </summary>
+        public (float xxa, float zza) GetMoveVector()
+        {
+            float xxa = 0;
+            float zza = 0;
+
+            if (Forward) zza += 1.0f;
+            if (Back) zza -= 1.0f;
+            if (Left) xxa += 1.0f;
+            if (Right) xxa -= 1.0f;
+
+            float lenSqr = xxa * xxa + zza * zza;
+            if (lenSqr > 1.0f)
+            {
+                float len = MathF.Sqrt(lenSqr);
+                xxa /= len;
+                zza /= len;
+            }
+
+            return (xxa, zza);
+        }
+
+        public void Reset()
+        {
+            Forward = false;
+            Back = false;
+            Left = false;
+            Right = false;
+            Jump = false;
+            Sneak = false;
+            Sprint = false;
+        }
+    }
+}