Migrate JSON stack to System.Text.Json (drop Newtonsoft.Json)

[tig]

Tracks the "JSON stack" open item in PLAN.md §5.

Tasks

• Audit whether the v2 code still references Newtonsoft.Json (the dead Serializers.cs using TypeNameHandling.All was already removed in Updates OCGV and SHOT to Terminal.Gui v2 PowerShell/ConsoleGuiTools#267).
• If present, migrate remaining usage to System.Text.Json.
• Drop the Newtonsoft.Json package reference from Directory.Packages.props and project files.

Legacy-cruft cleanup; pairs with the de-Microsoft rebrand.

[tig]

Done on develop in 2031bdf.

The entire Newtonsoft footprint was a single [JsonIgnore] on DataTableColumn.Type (plus a // TODO: switch to System.Text.Json). System.Text.Json.Serialization exposes the same JsonIgnore attribute and ships in the net10 BCL, so:

• using Newtonsoft.Json; → using System.Text.Json.Serialization;
• Dropped the Newtonsoft.Json PackageReference (PSTui.Models) and PackageVersion (Directory.Packages.props) — no replacement package needed.

grep confirms zero Newtonsoft references remain. The dead Serializers.cs that previously used TypeNameHandling.All was already removed in PowerShell#267. Closing once CI (#1) confirms the build.

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Generated by Claude Code

[tig]

✅ Confirmed green. CI now runs on develop (build + xUnit) across windows/macos/ubuntu — 78/78 tests pass on .NET 10 with the System.Text.Json migration in place. grep shows zero Newtonsoft references remaining. Run: https://github.com/gui-cs/PSTui/actions/runs/28041156533