feat(spawners): virtual OnTick; SpawnerDto carries the group flag (#2640)

## Summary

Two small additive changes a derived spawner needs.

- **`BaseSpawner.OnTick()` is now `virtual`.** A subclass that gates spawning on external state (time windows, event triggers) must gate *timer* spawns without gating the manual `Spawn()` API, and `OnTick` is the only place the two paths differ: it is the timer callback and `Spawn()` is both what it calls and what commands and scripts call. Cost: one virtual dispatch on the existing timer callback; no change to stock behaviour.
- **`group` in the JSON DTO.** `BaseSpawner.Group` (all dead, then respawn) is binary-persisted but was missing from `SpawnerDto`, so it did not survive export/import. Added to the abstract record after `spawnLocationIsHome`, assigned in `ApplyDto` after `InitSpawn` (which resets it), and exported by the three stock `ToDto` implementations.

## Test plan

- [x] A derived spawner overriding `OnTick` with a closed gate: `OnTick()` spawns nothing; manual `Spawn()` still spawns and does not pass through `OnTick`.
- [x] DTO round trip with `Group = true` carries `group` and restores it on `ToSpawner()`.
- [x] `UOContent.Tests` full suite green.
- [ ] CI

Projects/UOContent.Tests/Tests/Engines/Spawners/SpawnerTickAndGroupDtoTests.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections.Generic;
+using System.Text.Json;
+using Server.Engines.Spawners;
+using Xunit;
+
+namespace Server.Tests.Engines.Spawners;
+
+[Collection("Sequential UOContent Tests")]
+public class SpawnerTickAndGroupDtoTests
+{
+    private sealed class GatedSpawner : Spawner
+    {
+        public int Ticks;
+        public bool Gate = false;
+
+        public GatedSpawner() : base(1, TimeSpan.FromMinutes(5), TimeSpan.FromMinutes(10), 0, default, "Rabbit")
+        {
+        }
+
+        public GatedSpawner(Serial serial) : base(serial)
+        {
+        }
+
+        public override void OnTick()
+        {
+            Ticks++;
+            if (Gate)
+            {
+                base.OnTick();
+            }
+        }
+    }
+
+    [Fact]
+    public void OnTick_IsVirtual_AndManualSpawnBypassesIt()
+    {
+        var spawner = new GatedSpawner();
+        spawner.MoveToWorld(new Point3D(1500, 1500, 0), Map.Felucca);
+        try
+        {
+            spawner.OnTick();
+            Assert.Equal(1, spawner.Ticks);
+            Assert.Empty(spawner.Spawned);      // gate closed: base.OnTick not reached
+
+            spawner.Spawn();                    // manual API does not go through OnTick
+            Assert.Equal(1, spawner.Ticks);
+            Assert.Single(spawner.Spawned);
+        }
+        finally
+        {
+            spawner.Delete();
+        }
+    }
+
+    [Fact]
+    public void Dto_RoundTrip_CarriesGroup()
+    {
+        var spawner = new Spawner(1, TimeSpan.FromMinutes(5), TimeSpan.FromMinutes(10), 0, default, "Rabbit");
+        spawner.MoveToWorld(new Point3D(1500, 1500, 0), Map.Felucca);
+        Spawner loaded = null;
+        try
+        {
+            spawner.Group = true;
+            var json = SpawnerJsonSerializer.SerializeCompact(new List<SpawnerDto> { spawner.ToDto() });
+            Assert.Contains("\"group\"", json);
+
+            var dtos = JsonSerializer.Deserialize<List<SpawnerDto>>(json, SpawnerJsonSerializer.Options);
+            loaded = (Spawner)dtos[0].ToSpawner();
+            Assert.True(loaded.Group);
+        }
+        finally
+        {
+            loaded?.Delete();
+            spawner.Delete();
+        }
+    }
+}

Projects/UOContent/Engines/Spawners/BaseSpawner.Dto.cs

@@ -37,6 +37,7 @@ public abstract partial class BaseSpawner
 
         InitSpawn(dto.Count, dto.MinDelay, dto.MaxDelay, dto.Team, SpawnBounds);
 
+        _group = dto.Group;
         _walkingRange = dto.WalkingRange;
         _spawnLocationIsHome = dto.SpawnLocationIsHome;
         _spawnPositionMode = dto.SpawnPositionMode;

Projects/UOContent/Engines/Spawners/BaseSpawner.cs

@@ -822,7 +822,11 @@ public abstract partial class BaseSpawner : Item, ISpawner
         return remove && Spawned.Remove(spawned);
     }
 
-    public void OnTick()
+    /// <summary>
+    /// Timer callback. Override to gate or reorder tick work; manual <see cref="Spawn"/> does not
+    /// pass through here.
+    /// </summary>
+    public virtual void OnTick()
     {
         if (_group)
         {

Projects/UOContent/Engines/Spawners/Json/SpawnerDto.cs

@@ -78,12 +78,19 @@ public abstract record SpawnerDto
     [JsonPropertyOrder(11)]
     public bool SpawnLocationIsHome { get; init; }
 
-    [JsonPropertyName("spawnPositionMode")]
+    // Order values are unique on purpose: STJ's JsonPropertyOrder ties are documented as
+    // undefined between equal values, so every field gets its own slot.
+    /// <summary>All dead, then respawn together. Mirrors <see cref="BaseSpawner.Group"/>.</summary>
+    [JsonPropertyName("group")]
     [JsonPropertyOrder(12)]
+    public bool Group { get; init; }
+
+    [JsonPropertyName("spawnPositionMode")]
+    [JsonPropertyOrder(13)]
     public SpawnPositionMode SpawnPositionMode { get; init; }
 
     [JsonPropertyName("maxSpawnAttempts")]
-    [JsonPropertyOrder(13)]
+    [JsonPropertyOrder(14)]
     public int MaxSpawnAttempts { get; init; }
 
     // Compact square-bounds form, -1 when absent. Written only when >= 0 (ShouldSerialize, since 0
@@ -195,15 +202,15 @@ public sealed record ProximitySpawnerDto : SpawnerDto
     public Rectangle3D SpawnBounds { get; init; }
 
     [JsonPropertyName("triggerRange")]
-    [JsonPropertyOrder(14)]
+    [JsonPropertyOrder(15)]
     public int TriggerRange { get; init; }
 
     [JsonPropertyName("spawnMessage")]
-    [JsonPropertyOrder(15)]
+    [JsonPropertyOrder(16)]
     public TextDefinition SpawnMessage { get; init; }
 
     [JsonPropertyName("instant")]
-    [JsonPropertyOrder(16)]
+    [JsonPropertyOrder(17)]
     public bool Instant { get; init; }
 
     [JsonPropertyName("entries")]

Projects/UOContent/Engines/Spawners/ProximitySpawner.Dto.cs

@@ -33,6 +33,7 @@ public partial class ProximitySpawner
             WalkingRange = DtoWalkingRange,
             Entries = EntryList ?? [],
             SpawnLocationIsHome = SpawnLocationIsHome,
+            Group = Group,
             SpawnPositionMode = DtoSpawnPositionMode,
             MaxSpawnAttempts = DtoMaxSpawnAttempts,
             HomeRange = homeRange,

Projects/UOContent/Engines/Spawners/RegionSpawner.Dto.cs

@@ -31,6 +31,7 @@ public partial class RegionSpawner
         WalkingRange = DtoWalkingRange,
         Entries = EntryList ?? [],
         SpawnLocationIsHome = SpawnLocationIsHome,
+        Group = Group,
         SpawnPositionMode = DtoSpawnPositionMode,
         MaxSpawnAttempts = DtoMaxSpawnAttempts,
         Region = SpawnRegion?.Name

Projects/UOContent/Engines/Spawners/Spawner.Dto.cs

@@ -33,6 +33,7 @@ public partial class Spawner
             WalkingRange = DtoWalkingRange,
             Entries = EntryList ?? [],
             SpawnLocationIsHome = SpawnLocationIsHome,
+            Group = Group,
             SpawnPositionMode = DtoSpawnPositionMode,
             MaxSpawnAttempts = DtoMaxSpawnAttempts,
             HomeRange = homeRange,