Feature: per-tower construction-phase build bisuals

Adding the correct pipeline for creating the different building phases for towers. Adding a default set with primitive cubes.

Assets/_Project/Scripts/Towers/ConstructionPhaseSet.cs

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+// Assets/_Project/Scripts/Towers/ConstructionPhaseSet.cs
+using UnityEngine;
+
+namespace TD.Towers
+{
+    /// <summary>
+    /// An ordered set of construction-stage visuals a build site transitions between as a
+    /// tower is built — typically 3 assets (early → mid → late). A tower references its own
+    /// set via <see cref="TowerDefinition.ConstructionPhases"/>; build sites fall back to a
+    /// project-default set when a tower doesn't define one.
+    /// </summary>
+    /// <remarks>
+    /// The entries are <b>shape</b> prefabs only. <c>BuildSiteVisual</c> applies the
+    /// queued (green) / constructing / paused materials and the owner tint on top, so the
+    /// phase assets don't need their own build-state materials. Keep them simple
+    /// (single material) — the build site swaps the active phase's renderer material.
+    /// </remarks>
+    [CreateAssetMenu(fileName = "ConstructionPhaseSet", menuName = "TD/Construction Phase Set")]
+    public class ConstructionPhaseSet : ScriptableObject
+    {
+        [Tooltip("Ordered construction-stage prefabs. Shown in sequence as the build " +
+                 "progresses — e.g. 3 entries = early / mid / late thirds of the build.")]
+        [SerializeField] private GameObject[] phases;
+
+        /// <summary>Number of construction phases in this set.</summary>
+        public int PhaseCount => phases != null ? phases.Length : 0;
+
+        /// <summary>The phase prefab at <paramref name="index"/>, or null if out of range.</summary>
+        public GameObject GetPhase(int index)
+            => (phases != null && index >= 0 && index < phases.Length) ? phases[index] : null;
+    }
+}