refactor: fold hand-written serializable property setters into field hooks (#2587)

## Summary

Folds **113** hand-written `[SerializableProperty]` members into plain `[SerializableField]` declarations using the v4 setter hooks — value coercion/vetoes via `allowFieldChange`, post-change side effects via `fieldChanged` (whose `oldValue` parameter covers the old-house/old-sender unsubscribe patterns). Net **-450 lines** of setter boilerplate.

```cs
// before
[SerializableProperty(1)]
[CommandProperty(AccessLevel.GameMaster)]
public int Charges
{
    get => _charges;
    set
    {
        _charges = Math.Clamp(value, 0, MaxCharges);
        InvalidateProperties();
        this.MarkDirty();
    }
}

// after
[SerializableField(1, allowFieldChange: nameof(AllowChargesChange))]
[SerializedCommandProperty(AccessLevel.GameMaster)]
[InvalidateProperties]
private int _charges;

private bool AllowChargesChange(ref int value)
{
    value = Math.Clamp(value, 0, MaxCharges);
    return true;
}
```

## How sites were selected

A classifier parsed all 204 `[SerializableProperty]` sites and converted only those matching strict shapes: getter is exactly `get => _field;`, the assignment comes first (after at most an equality guard), and relocated side effects contain no `return`, no `value` mutation, and no field re-assignment. Everything else was left alone deliberately:

- **~34 custom getters** (fallback defaults like `_x == -1 ? Default : _x`, self-healing refs) — no setter hook can express these.
- **~35 pre-assignment logic** (durability Unscale/Scale sandwiches, old-state captures like PotionKeg's pile weight).
- **virtual/override members, name-mismatched backing fields (`m_`), exotic semantics** (guards' `Focus` does work on *equal* assignment; `ChampionSpawn.Active` never assigns its field).

Five sites the classifier refused were converted by hand where the hooks fit cleanly: `ReceiverCrystal.Sender`, `PlayerVendor.House`, `PlayerBarkeeper.House` (old-value unsubscribe via `oldValue`), `BaseSuit.AccessLevel` (its existing virtual `OnAccessLevelChanged` already had the exact callback shape), and `DyeTub.DyedHue` (a true veto: `AllowDyedHueChange(ref int value) => _redyable`).

## Verification

- Build: **0 errors, 0 warnings**.
- **Schema regeneration produces zero Migrations changes** — the conversion is wire- and schema-neutral by construction (same orders, types, and property names), and CI's schema diff check enforces it.
- **835 + 708 tests green.**

## Behavioral notes (all strict improvements, called out for review)

- Generated setters skip everything when the incoming value equals the current one; a few converted setters previously re-ran side effects on equal assignment (redundant `Update()`-style refreshes).
- Generated setters always `MarkDirty()` on change; several converted setters never did (e.g. `DyeTub.DyedHue`, `MorphItem` ranges) — their changes only persisted if something else dirtied the entity. Those latent persistence bugs are fixed by construction.

Projects/UOContent/Engines/CannedEvil/ChampionSkullBrazier.cs

@@ -33,17 +33,13 @@ public partial class ChampionSkullBrazier : AddonComponent
     [SerializedCommandProperty(AccessLevel.GameMaster)]
     private ChampionSkullPlatform _platform;
 
-    [SerializableProperty(2)]
-    [CommandProperty(AccessLevel.GameMaster)]
-    public Item Skull
+    [SerializableField(2, fieldChanged: nameof(OnSkullChanged))]
+    [SerializedCommandProperty(AccessLevel.GameMaster)]
+    private Item _skull;
+
+    private void OnSkullChanged(Item oldValue, Item newValue)
     {
-        get => _skull;
-        set
-        {
-            _skull = value;
-            this.MarkDirty();
-            _platform?.Validate();
-        }
+        _platform?.Validate();
     }
 
     public override int LabelNumber => 1049489 + (int)_type;

Projects/UOContent/Engines/CannedEvil/ChampionSpawn.cs

@@ -203,47 +203,38 @@ public partial class ChampionSpawn : Item
         }
     }
 
-    [SerializableProperty(3)]
-    [CommandProperty(AccessLevel.GameMaster, AccessLevel.Administrator)]
-    public int MaxLevel
+    [SerializableField(3, allowFieldChange: nameof(AllowMaxLevelChange))]
+    [SerializedCommandProperty(AccessLevel.GameMaster, AccessLevel.Administrator)]
+    private int _maxLevel;
+
+    private bool AllowMaxLevelChange(ref int value)
     {
-        get => _maxLevel;
-        set => _maxLevel = Math.Clamp(value, 0, 18);
+        value = Math.Clamp(value, 0, 18);
+        return true;
     }
 
-    [SerializableProperty(9)]
-    [CommandProperty(AccessLevel.GameMaster)]
-    public Rectangle2D SpawnArea
+    [SerializableField(9, fieldChanged: nameof(OnSpawnAreaChanged))]
+    [SerializedCommandProperty(AccessLevel.GameMaster)]
+    [InvalidateProperties]
+    private Rectangle2D _spawnArea;
+
+    private void OnSpawnAreaChanged(Rectangle2D oldValue, Rectangle2D newValue)
     {
-        get => _spawnArea;
-        set
-        {
-            _spawnArea = value;
-            this.MarkDirty();
-            InvalidateProperties();
-            UpdateRegion();
-        }
+        UpdateRegion();
     }
 
-    [SerializableProperty(11)]
-    [CommandProperty(AccessLevel.GameMaster)]
-    public int Kills
+    [SerializableField(11, fieldChanged: nameof(OnKillsChanged))]
+    [SerializedCommandProperty(AccessLevel.GameMaster)]
+    [InvalidateProperties]
+    private int _kills;
+
+    private void OnKillsChanged(int oldValue, int newValue)
     {
-        get => _kills;
-        set
+        var n = _kills / (double)MaxKills;
+        var p = (int)(n * 100);
+        if (p < 90)
         {
-            _kills = value;
-            this.MarkDirty();
-
-            var n = _kills / (double)MaxKills;
-            var p = (int)(n * 100);
-
-            if (p < 90)
-            {
-                SetWhiteSkullCount(p / 20);
-            }
-
-            InvalidateProperties();
+            SetWhiteSkullCount(p / 20);
         }
     }