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/Server/Mobiles/Mods/SkillMod.cs

@@ -21,33 +21,29 @@ namespace Server;
 [SerializationGenerator(0)]
 public abstract partial class SkillMod : MobileMod
 {
-    [SerializableField(0)]
+    [SerializableField(0, fieldChanged: nameof(OnObeyCapChanged))]
     private bool _obeyCap;
 
-    [SerializableFieldChanged(0)]
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private void OnObeCapChanged(bool oldValue, bool newValue) => Owner?.Skills[_skill]?.Update();
+    private void OnObeyCapChanged(bool oldValue, bool newValue) => Owner?.Skills[_skill]?.Update();
 
-    [SerializableField(1)]
+    [SerializableField(1, fieldChanged: nameof(OnSkillChanged))]
     private SkillName _skill;
 
-    [SerializableFieldChanged(1)]
     private void OnSkillChanged(SkillName oldValue, SkillName newValue)
     {
         Owner?.Skills[newValue]?.Update();
         Owner?.Skills[oldValue]?.Update();
     }
 
-    [SerializableField(2)]
+    [SerializableField(2, fieldChanged: nameof(OnRelativeChanged))]
     private bool _relative;
 
-    [SerializableFieldChanged(2)]
     private void OnRelativeChanged(bool oldValue, bool newValue) => Owner?.Skills[_skill]?.Update();
 
-    [SerializableField(3)]
+    [SerializableField(3, fieldChanged: nameof(OnValueChanged))]
     private double _value;
 
-    [SerializableFieldChanged(3)]
     private void OnValueChanged(double oldValue, double newValue) => Owner?.Skills[_skill]?.Update();
 
     public SkillMod(Mobile owner) : base(owner)