[STJ-5] Migrate Fallout.Tooling.Generator JSON model (blocked on STJ-3) #118

Sub-task of #83 (Newtonsoft.Json → System.Text.Json migration). Blocked on STJ-3 (utility-lib rewrite).

Scope

Migrate Fallout.Tooling.Generator's JSON model + serializer from Newtonsoft to System.Text.Json. This is the code generator that reads the 62 tool .json definitions and emits the corresponding .Generated.cs files.

Files (≈6)

• src/Fallout.Tooling.Generator/Generators/ToolGenerator.cs
• src/Fallout.Tooling.Generator/Model/DataClass.cs
• src/Fallout.Tooling.Generator/Model/Enumeration.cs
• src/Fallout.Tooling.Generator/Model/Property.cs
• src/Fallout.Tooling.Generator/Model/Task.cs
• src/Fallout.Tooling.Generator/Model/Tool.cs
• src/Fallout.Tooling.Generator/ToolSerializer.cs

The model classes use [JsonProperty] attributes; they all need to become [JsonPropertyName]. The ToolSerializer uses JsonConvert.DeserializeObject and probably custom converters / contract resolvers.

Verification angle

./build.ps1 GenerateTools should produce byte-identical output for the 62 existing tool wrappers before vs after the migration. That's the strongest regression guarantee.

Acceptance

• All listed files migrated
• ./build.ps1 GenerateTools produces identical .Generated.cs files for all 62 wrappers
• Newtonsoft.Json PackageReference removed from Fallout.Tooling.Generator.csproj
• ./build.ps1 Test green