refactor: add IItemUpdater strategies and ItemUpdaterFactory

Introduce the Strategy + Factory design: IItemUpdater (Name + Update),
an ItemUpdaterBase with the single shared [0,50] clamp helper, stateless
strategies for normal/Aged Brie/Sulfuras/Backstage, and a factory with
two-tier resolution (exact name via case-insensitive dictionary, then
Conjured prefix rule, then the default fallback).

Not wired into GildedRose yet; no behaviour change. The Conjured
strategy itself arrives in the feature commit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

csharp.xUnit/GildedRose/AgedBrieUpdater.cs

@@ -0,0 +1,16 @@
+namespace GildedRoseKata;
+
+public class AgedBrieUpdater : ItemUpdaterBase
+{
+    public override string Name => "Aged Brie";
+
+    public override void Update(Item item)
+    {
+        item.Quality = ClampQuality(item.Quality + 1);
+        item.SellIn -= 1;
+        if (item.SellIn < 0)
+        {
+            item.Quality = ClampQuality(item.Quality + 1);
+        }
+    }
+}

csharp.xUnit/GildedRose/BackstagePassUpdater.cs

@@ -0,0 +1,17 @@
+namespace GildedRoseKata;
+
+public class BackstagePassUpdater : ItemUpdaterBase
+{
+    public override string Name => "Backstage passes to a TAFKAL80ETC concert";
+
+    public override void Update(Item item)
+    {
+        var increase = item.SellIn <= 5 ? 3 : item.SellIn <= 10 ? 2 : 1;
+        item.Quality = ClampQuality(item.Quality + increase);
+        item.SellIn -= 1;
+        if (item.SellIn < 0)
+        {
+            item.Quality = 0;
+        }
+    }
+}

csharp.xUnit/GildedRose/IItemUpdater.cs

@@ -0,0 +1,10 @@
+namespace GildedRoseKata;
+
+public interface IItemUpdater
+{
+    /// <summary>The exact item name this strategy handles ("default" for the fallback).</summary>
+    string Name { get; }
+
+    /// <summary>Applies one full day: quality change + SellIn change + clamping.</summary>
+    void Update(Item item);
+}

csharp.xUnit/GildedRose/IItemUpdaterFactory.cs

@@ -0,0 +1,6 @@
+namespace GildedRoseKata;
+
+public interface IItemUpdaterFactory
+{
+    IItemUpdater Get(Item item);
+}

csharp.xUnit/GildedRose/ItemUpdaterBase.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace GildedRoseKata;
+
+public abstract class ItemUpdaterBase : IItemUpdater
+{
+    public const int MinQuality = 0;
+    public const int MaxQuality = 50;
+
+    public abstract string Name { get; }
+
+    public abstract void Update(Item item);
+
+    protected static int ClampQuality(int quality) => Math.Clamp(quality, MinQuality, MaxQuality);
+}

csharp.xUnit/GildedRose/ItemUpdaterFactory.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+
+namespace GildedRoseKata;
+
+public class ItemUpdaterFactory : IItemUpdaterFactory
+{
+    public const string ConjuredPrefix = "Conjured";
+    private const string DefaultName = "default";
+
+    private readonly Dictionary<string, IItemUpdater> _updatersByName;
+    private readonly IItemUpdater _fallback;
+
+    public ItemUpdaterFactory(IEnumerable<IItemUpdater> updaters)
+    {
+        _updatersByName = new Dictionary<string, IItemUpdater>(StringComparer.OrdinalIgnoreCase);
+        foreach (var updater in updaters)
+        {
+            _updatersByName[updater.Name] = updater;
+        }
+
+        if (!_updatersByName.TryGetValue(DefaultName, out _fallback))
+        {
+            throw new ArgumentException($"A \"{DefaultName}\" fallback updater is required.", nameof(updaters));
+        }
+    }
+
+    public static ItemUpdaterFactory Default() => new(new IItemUpdater[]
+    {
+        new NormalItemUpdater(),
+        new AgedBrieUpdater(),
+        new SulfurasUpdater(),
+        new BackstagePassUpdater(),
+    });
+
+    public IItemUpdater Get(Item item)
+    {
+        if (_updatersByName.TryGetValue(item.Name, out var updater))
+        {
+            return updater;
+        }
+
+        if (item.Name.StartsWith(ConjuredPrefix, StringComparison.OrdinalIgnoreCase) &&
+            _updatersByName.TryGetValue(ConjuredPrefix, out var conjured))
+        {
+            return conjured;
+        }
+
+        return _fallback;
+    }
+}

csharp.xUnit/GildedRose/NormalItemUpdater.cs

@@ -0,0 +1,16 @@
+namespace GildedRoseKata;
+
+public class NormalItemUpdater : ItemUpdaterBase
+{
+    public override string Name => "default";
+
+    public override void Update(Item item)
+    {
+        item.Quality = ClampQuality(item.Quality - 1);
+        item.SellIn -= 1;
+        if (item.SellIn < 0)
+        {
+            item.Quality = ClampQuality(item.Quality - 1);
+        }
+    }
+}

csharp.xUnit/GildedRose/SulfurasUpdater.cs

@@ -0,0 +1,11 @@
+namespace GildedRoseKata;
+
+public class SulfurasUpdater : ItemUpdaterBase
+{
+    public override string Name => "Sulfuras, Hand of Ragnaros";
+
+    public override void Update(Item item)
+    {
+        // Legendary item: Quality stays at 80 and SellIn never changes.
+    }
+}