feat: adopt serialization generator v4 (field-side linkage, anchored timers) (#2586)

## Summary

Adopts ModernUO.Serialization 4.0.0 across the engine. Three commits, reviewable independently:

1. **Package + tool bump to 4.0.0** (`Server.csproj`, `UOContent.csproj`, `dotnet-tools.json`).
2. **Timers → `[DeserializeTimer]`** — the 8 drifting timers (BaseLight, TreasureMapChest, MarkContainer, FillableContainer, DeathRobe, DecayedCorpse, Corpse, BaseEscortable) now store their next tick as **anchored time**: server downtime no longer consumes the remaining delay, and idle-world saves are byte-stable. This changes their wire format, so each class bumps its serialization version with a `MigrateFrom` that replays the old delta-time read through the migration schema (the new `vN.json` files carry `@AnchoredTimer`; the old ones keep `@TimerDrift`, which the generator reads forever). The 2 wall-clock timers (Aquarium, FountainOfLife) keep their exact format via `wallClock: true` — no bump. Restart methods drop their `TimeSpan.MinValue` sentinel checks: v4 invokes them **only when a timer was actually running at save**.
3. **Linkage → field-side declarations** — 175 conversions across 25 files: `[SerializableFieldSaveFlag(order)]`/`[SerializableFieldDefault(order)]` become `[SaveFlag(nameof(...), nameof(...))]` on the field, and `[SerializableFieldChanged(order)]` becomes the `fieldChanged:` argument of `[SerializableField]`. **Wire-neutral: zero migration schemas changed.**

## Verification

- Solution builds with **0 errors, 0 warnings**; all three 4.0.0 packages verified indexed on nuget.org (no local feed needed).
- **835 + 708 tests green.**
- Generated output inspected: old-version content structs replay `ReadDeltaTime` (e.g. `V3Content.DecayTimerNext = reader.ReadDeltaTime()`), current versions write/read anchored time with the gated restart, and the wall-clock classes emit byte-identical `Write`/`ReadDateTime` framing.
- Schema tool run is committed (CI's `git diff --exit-code` schema check passes): exactly the 8 expected new `vN.json` files, nothing else touched.
- The conversion was scripted with a class-scoped resolver (order → same-class `[SerializableField(order)]`/`[SerializableProperty(order)]`); it planned 175/175 with zero ambiguities before applying.

## Notes

- New `MigrateFrom`s use the content structs' provided `XxxDelay` property, matching the pre-existing idiom in Corpse's and TreasureMapChest's older migrations.
- Follow-up candidate (separate PR, wire-neutral, any time): fold the ~150 eligible hand-written `[SerializableProperty]` setters (clamps, post-change side effects) down to `[SerializableField]` with `allowFieldChange`/`fieldChanged` hooks.

Projects/UOContent/Migrations/Server.Items.Corpse.v18.json

@@ -0,0 +1,137 @@
+{
+  "version": 18,
+  "type": "Server.Items.Corpse",
+  "properties": [
+    {
+      "name": "RestoreEquip",
+      "type": "System.Collections.Generic.List\u003CServer.Item\u003E",
+      "rule": "ListMigrationRule",
+      "ruleArguments": [
+        "Server.Item",
+        "SerializableInterfaceMigrationRule"
+      ]
+    },
+    {
+      "name": "Flags",
+      "type": "Server.Items.CorpseFlag",
+      "rule": "EnumMigrationRule"
+    },
+    {
+      "name": "TimeOfDeath",
+      "type": "System.DateTime",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        "DeltaTime"
+      ]
+    },
+    {
+      "name": "RestoreTable",
+      "type": "System.Collections.Generic.Dictionary\u003CServer.Item, Server.Point3D\u003E",
+      "rule": "DictionaryMigrationRule",
+      "ruleArguments": [
+        "Server.Item",
+        "SerializableInterfaceMigrationRule",
+        "0",
+        "Server.Point3D",
+        "PrimitiveUOTypeMigrationRule",
+        "1",
+        "Point3D"
+      ]
+    },
+    {
+      "name": "DecayTimer",
+      "type": "Server.Timer",
+      "rule": "TimerMigrationRule",
+      "ruleArguments": [
+        "@AnchoredTimer"
+      ]
+    },
+    {
+      "name": "Looters",
+      "type": "System.Collections.Generic.HashSet\u003CServer.Mobile\u003E",
+      "rule": "HashSetMigrationRule",
+      "ruleArguments": [
+        "Server.Mobile",
+        "SerializableInterfaceMigrationRule"
+      ]
+    },
+    {
+      "name": "Killer",
+      "type": "Server.Mobile",
+      "rule": "SerializableInterfaceMigrationRule"
+    },
+    {
+      "name": "Aggressors",
+      "type": "System.Collections.Generic.List\u003CServer.Mobile\u003E",
+      "rule": "ListMigrationRule",
+      "ruleArguments": [
+        "Server.Mobile",
+        "SerializableInterfaceMigrationRule"
+      ]
+    },
+    {
+      "name": "Owner",
+      "type": "Server.Mobile",
+      "rule": "SerializableInterfaceMigrationRule"
+    },
+    {
+      "name": "CorpseName",
+      "type": "string",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "AccessLevel",
+      "type": "Server.AccessLevel",
+      "rule": "EnumMigrationRule"
+    },
+    {
+      "name": "Guild",
+      "type": "Server.Guilds.Guild",
+      "rule": "SerializableInterfaceMigrationRule"
+    },
+    {
+      "name": "EquipItems",
+      "type": "System.Collections.Generic.List\u003CServer.Item\u003E",
+      "rule": "ListMigrationRule",
+      "ruleArguments": [
+        "Server.Item",
+        "SerializableInterfaceMigrationRule"
+      ]
+    },
+    {
+      "name": "HairItemId",
+      "type": "int",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "HairHue",
+      "type": "int",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "FacialHairItemId",
+      "type": "int",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "FacialHairHue",
+      "type": "int",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    }
+  ]
+}