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/Engines/Plants/PlantItem.cs

@@ -37,17 +37,17 @@ public partial class PlantItem : Item, ISecurable
 
     [SerializedIgnoreDupe]
     [SerializableField(0)]
+    [SaveFlag(nameof(ShouldSerializeSecureLevel))]
     [SerializedCommandProperty(AccessLevel.GameMaster)]
     private SecureLevel _level;
 
-    [SerializableFieldSaveFlag(0)]
     private bool ShouldSerializeSecureLevel() => (int)_level != 0;
 
     [SerializedIgnoreDupe]
     [SerializableField(5, setter: "private")]
+    [SaveFlag(nameof(ShouldSerializePlantSystem))]
     private PlantSystem _plantSystem;
 
-    [SerializableFieldSaveFlag(5)]
     private bool ShouldSerializePlantSystem() => _plantStatus < PlantStatus.DecorativePlant;
 
     // For clients older than 7.0.12.0
@@ -82,6 +82,7 @@ public partial class PlantItem : Item, ISecurable
 
     [CommandProperty(AccessLevel.GameMaster)]
     [SerializableProperty(1)]
+    [SaveFlag(nameof(ShouldSerializePlantStatus))]
     public PlantStatus PlantStatus
     {
         get => _plantStatus;
@@ -120,10 +121,10 @@ public partial class PlantItem : Item, ISecurable
         }
     }
 
-    [SerializableFieldSaveFlag(1)]
     private bool ShouldSerializePlantStatus() => _plantStatus != PlantStatus.BowlOfDirt;
 
     [SerializableProperty(2)]
+    [SaveFlag(nameof(ShouldSerializePlantType))]
     [CommandProperty(AccessLevel.GameMaster)]
     public PlantType PlantType
     {
@@ -135,10 +136,10 @@ public partial class PlantItem : Item, ISecurable
         }
     }
 
-    [SerializableFieldSaveFlag(2)]
     private bool ShouldSerializePlantType() => (int)_plantType != 0;
 
     [SerializableProperty(3)]
+    [SaveFlag(nameof(ShouldSerializePlantHue))]
     [CommandProperty(AccessLevel.GameMaster)]
     public PlantHue PlantHue
     {
@@ -150,10 +151,10 @@ public partial class PlantItem : Item, ISecurable
         }
     }
 
-    [SerializableFieldSaveFlag(3)]
     private bool ShouldSerializePlantHue() => _plantHue != PlantHue.None;
 
     [SerializableProperty(4)]
+    [SaveFlag(nameof(ShouldSerializeShowType))]
     [CommandProperty(AccessLevel.GameMaster)]
     public bool ShowType
     {
@@ -166,7 +167,6 @@ public partial class PlantItem : Item, ISecurable
         }
     }
 
-    [SerializableFieldSaveFlag(4)]
     private bool ShouldSerializeShowType() => _showType;
 
     [CommandProperty(AccessLevel.GameMaster)]