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/Spawners/BaseSpawner.cs

@@ -54,10 +54,10 @@ public abstract partial class BaseSpawner : Item, ISpawner
     [SerializedCommandProperty(AccessLevel.Developer)]
     private Guid _guid;
 
-    [SerializableFieldSaveFlag(1)]
     private bool ShouldSerializeReturnOnDeactivate() => _returnOnDeactivate;
 
     [SerializableField(1)]
+    [SaveFlag(nameof(ShouldSerializeReturnOnDeactivate))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private bool _returnOnDeactivate;
 
@@ -67,48 +67,46 @@ public abstract partial class BaseSpawner : Item, ISpawner
 
     private int _walkingRange = -1;
 
-    [SerializableFieldSaveFlag(4)]
     private bool ShouldSerializeWayPoint() => _wayPoint != null;
 
     [SerializableField(4)]
+    [SaveFlag(nameof(ShouldSerializeWayPoint))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private WayPoint _wayPoint;
 
-    [SerializableFieldSaveFlag(5)]
     private bool ShouldSerializeGroup() => _group;
 
     [InvalidateProperties]
     [SerializableField(5)]
+    [SaveFlag(nameof(ShouldSerializeGroup))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private bool _group;
 
-    [SerializableFieldSaveFlag(6)]
     private bool ShouldSerializeMinDelay() => _minDelay != DefaultMinDelay;
 
-    [SerializableFieldDefault(6)]
     private TimeSpan MinDelayDefault() => DefaultMinDelay;
 
     [InvalidateProperties]
     [SerializableField(6)]
+    [SaveFlag(nameof(ShouldSerializeMinDelay), nameof(MinDelayDefault))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private TimeSpan _minDelay;
 
-    [SerializableFieldSaveFlag(7)]
     private bool ShouldSerializeMaxDelay() => _maxDelay != DefaultMaxDelay;
 
-    [SerializableFieldDefault(7)]
     private TimeSpan MaxDelayDefault() => DefaultMaxDelay;
 
     [InvalidateProperties]
     [SerializableField(7)]
+    [SaveFlag(nameof(ShouldSerializeMaxDelay), nameof(MaxDelayDefault))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private TimeSpan _maxDelay;
 
-    [SerializableFieldSaveFlag(9)]
     private bool ShouldSerializeTeam() => _team != 0;
 
     [InvalidateProperties]
     [SerializableField(9)]
+    [SaveFlag(nameof(ShouldSerializeTeam))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private int _team;
 
@@ -125,29 +123,29 @@ public abstract partial class BaseSpawner : Item, ISpawner
     /// If true, the home location of the spawn is the location where it spawned
     /// If false, the home location of the spawn is the location of the spawner
     /// </summary>
-    [SerializableFieldSaveFlag(11)]
     private bool ShouldSerializeSpawnLocationIsHome() => _spawnLocationIsHome;
 
     [InvalidateProperties]
     [SerializableField(11)]
+    [SaveFlag(nameof(ShouldSerializeSpawnLocationIsHome))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private bool _spawnLocationIsHome;
 
-    [SerializableFieldSaveFlag(12)]
     private bool ShouldSerializeEnd() => _end != default;
 
     [SerializableField(12)]
+    [SaveFlag(nameof(ShouldSerializeEnd))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private DateTime _end;
 
     /// <summary>
     /// Controls how spawn position optimization is handled.
     /// </summary>
-    [SerializableFieldSaveFlag(13)]
     private bool ShouldSerializeSpawnPositionMode() =>
         _spawnPositionMode is not SpawnPositionMode.Automatic and not SpawnPositionMode.Abandoned;
 
     [SerializableField(13)]
+    [SaveFlag(nameof(ShouldSerializeSpawnPositionMode))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private SpawnPositionMode _spawnPositionMode;
 
@@ -156,13 +154,12 @@ public abstract partial class BaseSpawner : Item, ISpawner
     /// <summary>
     /// Maximum number of random position attempts before engaging optimization.
     /// </summary>
-    [SerializableFieldSaveFlag(14)]
     private bool ShouldSerializeMaxSpawnAttempts() => _maxSpawnAttempts != DefaultMaxSpawnAttempts;
 
-    [SerializableFieldDefault(14)]
     private int MaxSpawnAttemptsDefault() => DefaultMaxSpawnAttempts;
 
     [SerializableField(14)]
+    [SaveFlag(nameof(ShouldSerializeMaxSpawnAttempts), nameof(MaxSpawnAttemptsDefault))]
     [SerializedCommandProperty(AccessLevel.Developer)]
     private int _maxSpawnAttempts;