[STJ-1] Replace NJsonSchema.NewtonsoftJson with NJsonSchema core (STJ) #114

Sub-task of #83 (Newtonsoft.Json → System.Text.Json migration).

Scope

Replace NJsonSchema.NewtonsoftJson with NJsonSchema core configured against System.Text.Json.

Files

• src/Fallout.Build/Utilities/SchemaUtility.cs — uses NewtonsoftJsonSchemaGeneratorSettings, DefaultContractResolver, Newtonsoft JsonConverter, StringEnumConverter. Needs the equivalent STJ-flavored API: SystemTextJsonSchemaGeneratorSettings (or whichever the current NJsonSchema package calls it), JsonTypeInfoResolver, STJ converters.
• src/Fallout.Common/Tools/NSwag/NSwag.Generated.cs — check whether the generated tool wrapper actually references Newtonsoft or just inherits via NJsonSchema's transitive package. May or may not need changes.

Acceptance

• NJsonSchema.NewtonsoftJson removed from Directory.Packages.props
• SchemaUtility.GetJsonString / GetJsonDocument still emit equivalent JSON for the build schema
• ./build.ps1 Test green

Notes

Independent of the utility-lib rewrite ([STJ-3 / #PLACEHOLDER]). Can be done in any order relative to the others.