feat(core): implement playable CTF prototype with 6 core scripts

Implements the minimal playable core for the teaser prototype:
- Game.cs: Bootstrap, scene setup, scoring, round reset, win condition
- Unit.cs: Movement, route following, tagging, respawn
- RouteDrawer.cs: Click-drag route input with LineRenderer preview
- Flag.cs: Pickup, drop, return mechanics
- Visibility.cs: Fog of war via SpriteRenderer visibility
- SimpleAI.cs: Enemy AI that chases player flag

All game objects created programmatically (no Editor setup required).
Uses RuntimeInitializeOnLoadMethod for automatic bootstrap.

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

Backyard CTF/Assets/Scripts/Visibility.cs

@@ -0,0 +1,59 @@
+using UnityEngine;
+
+public class Visibility : MonoBehaviour
+{
+    public Unit[] playerUnits;
+    public Unit[] enemyUnits;
+    public Flag enemyFlag;
+
+    void Update()
+    {
+        // Update visibility of enemy units
+        foreach (var enemy in enemyUnits)
+        {
+            if (enemy == null) continue;
+
+            bool visible = IsVisibleToAnyPlayerUnit(enemy.transform.position);
+            var sr = enemy.GetComponent<SpriteRenderer>();
+            if (sr != null)
+            {
+                sr.enabled = visible || enemy.isTaggedOut; // Show tagged out units (they're faded anyway)
+            }
+        }
+
+        // Update visibility of enemy flag (only when not carried)
+        if (enemyFlag != null && enemyFlag.carriedBy == null)
+        {
+            bool flagVisible = IsVisibleToAnyPlayerUnit(enemyFlag.transform.position);
+            var flagSr = enemyFlag.GetComponent<SpriteRenderer>();
+            if (flagSr != null)
+            {
+                flagSr.enabled = flagVisible;
+            }
+        }
+        else if (enemyFlag != null && enemyFlag.carriedBy != null)
+        {
+            // Flag is carried - always show it (it's attached to a visible unit)
+            var flagSr = enemyFlag.GetComponent<SpriteRenderer>();
+            if (flagSr != null)
+            {
+                flagSr.enabled = true;
+            }
+        }
+    }
+
+    bool IsVisibleToAnyPlayerUnit(Vector2 position)
+    {
+        foreach (var unit in playerUnits)
+        {
+            if (unit == null || unit.isTaggedOut) continue;
+
+            float distance = Vector2.Distance(unit.transform.position, position);
+            if (distance < Game.VisionRadius)
+            {
+                return true;
+            }
+        }
+        return false;
+    }
+}