refactor: Move legacy deserialization into the .Migrations.cs partials (#2575)

Follow-up to #2574, which added a `.Migrations.cs` partial to `BaseWeapon`. Pure relocation — no behaviour change.

## The inconsistency

`BaseArmor` and `BaseClothing` already kept their pre-codegen `Deserialize(reader, version)` in a `.Migrations.cs` partial, but left the `OldSaveFlag` enum and the `GetSaveFlag` helper behind in the main class file — even though every call site is in the partial:

| Class | `Deserialize` | `GetSaveFlag` / `OldSaveFlag` | Call sites outside the partial |
|---|---|---|---|
| `BaseArmor` | already in partial | in main file | 0 of 26 |
| `BaseClothing` | already in partial | in main file | 0 of 12 |
| `BaseWeapon` | in main file | in main file | — |

`BaseWeapon` had all three still inline, with its new `.Migrations.cs` holding only a `MigrateFrom`.

## After

All three follow the same layout: `MigrateFrom` newest to oldest, then the pre-codegen `Deserialize`, then `GetSaveFlag`, then `OldSaveFlag`. That moves ~290 lines of legacy read path out of `BaseWeapon.cs` — the file that needed it most at ~3,900 lines — and leaves the main class files describing only how the type behaves today.

## Reviewing this

The diff is large and almost entirely noise, so it is probably not worth reading line by line. Two checks are stronger:

- **Nothing was lost or altered.** Across each `.cs` / `.Migrations.cs` pair, the multiset of non-blank source lines is identical to `main` except for one added comment (below). The relocation was done mechanically and asserted against that invariant rather than by hand.
- **Nothing about serialization moved with the code.** Running `ModernUOSchemaGenerator` after the move emits no new migration files.

The complete set of intentional additions:

- `using System;` in each of the three partials, for the `[Flags]` attribute (implicit usings are not enabled here).
- `// Version 9 (pre-codegen)` above `BaseWeapon`'s moved `Deserialize`, matching the marker `BaseArmor` and `BaseClothing` already carry. Version 9 is correct because `BaseWeapon.v10.json` is its earliest migration schema, so codegen began at 10.

Everything else is blank-line placement.

## Verification

Full solution builds in Release with 0 errors and 0 warnings; 1516 tests pass (815 `Server.Tests`, 701 `UOContent.Tests`).

Projects/UOContent/Items/Clothing/BaseClothing.Migrations.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace Server.Items;
 
 public partial class BaseClothing
@@ -101,4 +103,23 @@ public partial class BaseClothing
 
         PlayerConstructed = GetSaveFlag(flags, OldSaveFlag.PlayerConstructed);
     }
+
+    private static bool GetSaveFlag(OldSaveFlag flags, OldSaveFlag toGet) => (flags & toGet) != 0;
+
+    [Flags]
+    private enum OldSaveFlag
+    {
+        None = 0x00000000,
+        Resource = 0x00000001,
+        Attributes = 0x00000002,
+        ClothingAttributes = 0x00000004,
+        SkillBonuses = 0x00000008,
+        Resistances = 0x00000010,
+        MaxHitPoints = 0x00000020,
+        HitPoints = 0x00000040,
+        PlayerConstructed = 0x00000080,
+        Crafter = 0x00000100,
+        Quality = 0x00000200,
+        StrReq = 0x00000400
+    }
 }

Projects/UOContent/Items/Clothing/BaseClothing.cs

@@ -880,8 +880,6 @@ namespace Server.Items
             InvalidateProperties();
         }
 
-        private static bool GetSaveFlag(OldSaveFlag flags, OldSaveFlag toGet) => (flags & toGet) != 0;
-
         [AfterDeserialization]
         private void AfterDeserialization()
         {
@@ -902,21 +900,5 @@ namespace Server.Items
             }
         }
 
-        [Flags]
-        private enum OldSaveFlag
-        {
-            None = 0x00000000,
-            Resource = 0x00000001,
-            Attributes = 0x00000002,
-            ClothingAttributes = 0x00000004,
-            SkillBonuses = 0x00000008,
-            Resistances = 0x00000010,
-            MaxHitPoints = 0x00000020,
-            HitPoints = 0x00000040,
-            PlayerConstructed = 0x00000080,
-            Crafter = 0x00000100,
-            Quality = 0x00000200,
-            StrReq = 0x00000400
-        }
     }
 }