[STJ-3] BREAKING: rewrite Fallout.Utilities.Text.Json on System.Text.Json

[ChrisonSimtian]

Sub-task of #83 (Newtonsoft.Json → System.Text.Json migration). Breaking change for downstream consumers.

Scope

Rewrite src/Fallout.Utilities.Text.Json to use System.Text.Json instead of Newtonsoft. This changes the public API surface of Fallout.Utilities.Text.Json — the consumer-facing extension methods currently return Newtonsoft JObject and accept Newtonsoft JsonSerializerSettings; they would return STJ JsonObject and accept JsonSerializerOptions instead.

Files to rewrite

• JsonExtensions.cs — DefaultSerializerSettings → DefaultSerializerOptions; ToJson / GetJson / ReadJson / WriteJson / UpdateJson signatures updated; type aliases swap (JObject ↔ JsonObject)
• JObject.GetChildren.cs → JsonObject.GetChildren.cs
• JObject.GetPropertyValue.cs → JsonObject.GetPropertyValue.cs
• Object.ToJObject.cs → Object.ToJsonNode.cs (or similar)
• Base64JsonConverter.cs — internal JsonConvert.DeserializeObject<T> → JsonSerializer.Deserialize<T>
• AllWritableContractResolver.cs — Newtonsoft-specific concept (DefaultContractResolver). May be deletable; if needed, replace with JsonTypeInfo modifier (STJ source-gen approach).

Blocks

Once STJ-3 lands, the following can be unblocked:

• STJ-4 (Fallout.Tooling migration — heavy user of utility-lib APIs)
• STJ-5 (Fallout.Tooling.Generator)
• Remaining utility-lib consumers from STJ-2

Acceptance

• Six files rewritten
• Newtonsoft.Json PackageReference removed from Fallout.Utilities.Text.Json.csproj
• All in-repo consumers of the utility lib updated to the new API in the same PR (or follow-up PRs that land before the major release)
• Breaking-change entry added to migration guide / changelog
• Decision recorded on whether to ship as 11.0 (breaking-change major) or differently

Notes

The earlier attempt at this (commit 7f6125, branch feature/migrate-to-system-text-json) showed 26 cascading compile errors in consumers. Each is mechanical to fix, but the work needs proper time-budget — see the scoping summary in PR #111 for the full assessment.