feat(grid): Replace real-time CTF with turn-based grid system

Replace continuous free-form movement with discrete 20x50 grid-based
gameplay featuring asymmetric movement mechanics:

- Blue/Red teams with 3 units each
- Zone-based movement: orthogonal (4 dir) in offense, diagonal (8 dir) in defense
- Alternating turns with click-to-select, click-to-move input
- Fog of war: 3-cell Chebyshev vision radius per unit
- Defense speed nerf: skip every 4th move in own zone
- AI opponent that chases flag carriers and advances toward enemy flag
- Collision resolution: defender wins in their zone, lower ID wins in neutral

Implements all 3 phases from the plan:
- Phase 1: Playable grid with hot-seat two-player
- Phase 2: Fog of war + defense speed nerf
- Phase 3: AI opponent

Deleted obsolete files: Flag.cs, Unit.cs, RouteDrawer.cs, SimpleAI.cs, Visibility.cs

Co-Authored-By: Claude <noreply@anthropic.com>

Backyard CTF/Assets/Scripts/Grid/GridUnit.cs

@@ -0,0 +1,37 @@
+using UnityEngine;
+
+public enum Team { Blue, Red }
+
+public class GridUnit
+{
+    public int UnitId;
+    public Team Team;
+    public Vector2Int GridPosition;
+    public int ConsecutiveDefenseMoves;
+    public bool IsTaggedOut;
+    public int RespawnTurnsRemaining;
+
+    // Visual representation
+    public GameObject GameObject;
+    public SpriteRenderer SpriteRenderer;
+
+    public GridUnit(int unitId, Team team, Vector2Int position, GameObject go)
+    {
+        UnitId = unitId;
+        Team = team;
+        GridPosition = position;
+        ConsecutiveDefenseMoves = 0;
+        IsTaggedOut = false;
+        RespawnTurnsRemaining = 0;
+        GameObject = go;
+        SpriteRenderer = go.GetComponent<SpriteRenderer>();
+    }
+
+    public void SetWorldPosition(Vector2 worldPos)
+    {
+        if (GameObject != null)
+        {
+            GameObject.transform.position = new Vector3(worldPos.x, worldPos.y, 0);
+        }
+    }
+}