Implements Task 2 of the spawner STJ migration:
- SpawnerJsonSerializer: assembly discovery of [JsonDiscoverableType]-marked
BaseSpawner subclasses, STJ polymorphism on $type, property pruning to
[JsonPropertyName]-only, and OnDeserialized→OnAfterJsonDeserialize() hook.
- BaseSpawner.Json.cs: shadow JSON properties for all BaseSpawner fields,
OnAfterJsonDeserialize() virtual method that applies deserialized state.
- Spawner.Json.cs: JsonSpawnBounds shadow property, override of
OnAfterJsonDeserialize() to apply spawnBounds after base.
- Spawner.cs: [JsonDiscoverableType] + [JsonConstructor] on parameterless ctor.
- TestServerInitializer: SpawnerJsonSerializer.Configure() wired in.
- SpawnerRoundTripTests: 2 tests (round-trip core fields, omits domain defaults).
Old DynamicJson ctors and ToJson() left in place; removal is Task 9.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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