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/Flag.cs

@@ -0,0 +1,97 @@
+using System.Collections;
+using UnityEngine;
+
+public class Flag : MonoBehaviour
+{
+    public Unit.Team team;
+    public Transform homePosition;
+    public Unit carriedBy;
+
+    float dropTimer;
+    bool isDropped;
+    Coroutine returnCoroutine;
+
+    void Update()
+    {
+        if (carriedBy != null)
+        {
+            // Follow carrier
+            transform.position = carriedBy.transform.position + new Vector3(0.3f, 0.3f, 0);
+        }
+    }
+
+    public void Pickup(Unit unit)
+    {
+        if (carriedBy != null) return;
+
+        // Cancel return timer if picking up dropped flag
+        if (returnCoroutine != null)
+        {
+            StopCoroutine(returnCoroutine);
+            returnCoroutine = null;
+        }
+
+        carriedBy = unit;
+        unit.hasFlag = true;
+        isDropped = false;
+
+        Debug.Log($"{unit.team} picked up {team} flag!");
+    }
+
+    public void Drop()
+    {
+        if (carriedBy == null) return;
+
+        carriedBy.hasFlag = false;
+        carriedBy = null;
+        isDropped = true;
+
+        Debug.Log($"{team} flag dropped!");
+
+        // Start return timer
+        returnCoroutine = StartCoroutine(ReturnAfterDelay());
+    }
+
+    IEnumerator ReturnAfterDelay()
+    {
+        yield return new WaitForSeconds(Game.FlagReturnDelay);
+
+        if (isDropped && carriedBy == null)
+        {
+            ReturnHome();
+        }
+    }
+
+    public void ReturnHome()
+    {
+        if (returnCoroutine != null)
+        {
+            StopCoroutine(returnCoroutine);
+            returnCoroutine = null;
+        }
+
+        if (carriedBy != null)
+        {
+            carriedBy.hasFlag = false;
+            carriedBy = null;
+        }
+
+        transform.position = homePosition.position;
+        isDropped = false;
+
+        Debug.Log($"{team} flag returned home!");
+    }
+
+    void OnTriggerEnter2D(Collider2D other)
+    {
+        // If flag is dropped and a friendly unit touches it, return it home immediately
+        if (isDropped && carriedBy == null)
+        {
+            var unit = other.GetComponent<Unit>();
+            if (unit != null && unit.team == team && !unit.isTaggedOut)
+            {
+                ReturnHome();
+            }
+        }
+    }
+}