feat(stjm): support .NET 8 and .NET 9 without JSON upgrades

[LiteracyFanatic]

.NET 8 and .NET 9 applications currently cannot consume the migration package. Adding a newer System.Text.Json package would also force a dependency upgrade in hosts that already load JSON 8. In a Civil 3D 2025 host test, that approach failed with FileLoadException for System.Text.Json 10.0.0.0; the new net8.0 asset passed using the host's existing JSON 8 assembly.

Add net8.0 and net9.0 alongside net10.0 and net11.0 across the migration library, tests, samples, and benchmarks. The only runtime compatibility branch reads JsonTypeInfo.ElementType through the existing StjInternals UnsafeAccessor pattern on .NET 8, where this property is internal. Reading the resolved JSON contract preserves collection routing semantics, including inherited collections with multiple IEnumerable interfaces, without duplicating System.Text.Json's converter-selection rules or adding a runtime package dependency. The .NET 9 and newer assets use the public property path; union support remains .NET 11-only.

Add 36 collection-routing cases and separate resolver snapshot retention, newer-runtime decorator retention, and actual cycle termination on .NET 8/9. Document the snapshot-before-decoration pattern and keep linked compatibility guidance consistent. Extend CI to install .NET 8 and .NET 9, run all supported targets, and verify packaged analyzers and trimming/NativeAOT diagnostics on each target. The analyzer package check explicitly loops over consumer frameworks; multi-target solution builds and tests are handled by MSBuild. Disable CheckEolTargetFramework within the migration package and each isolated verification consumer so newer SDKs do not reject the retained targets after their support end dates. The package-level props import the repository defaults; other packages retain their existing policy.

Validation:

• Release solution build: zero warnings/errors. NuGet package validation and documentation snippet verification passed.
• After the end-of-support property change, the Release solution build remained warning-free. Simulated SDK end-of-support checks passed for .NET 8/9 package projects and all four generated consumer templates; controls with CheckEolTargetFramework=true failed with NETSDK1138. Verified that all package projects still inherit repository defaults.
• Runtime tests: 395 each on .NET 8/9/10, 513 on .NET 11.
• Analyzer tests: 144 each on .NET 8/9/10 and 174 on .NET 11; sample tests: 44 each on .NET 8/9/10 and 49 on .NET 11. All passed.
• A package consumer built and ran successfully with .NET SDK 8.0.319 using JSON 8.
• Packed analyzer consumers passed on all four targets, including diagnostic severity checks. Untrimmed consumers ran, trimmed publish diagnostics passed, and NativeAOT compilation diagnostics passed. Local Windows AOT validation omitted native linking; the existing Linux CI path runs full publish.
• Civil 3D 2025 Core Console, .NET 8.0.29: loaded pvsite first, then a consumer of the packed net8.0 asset. Static migration, tracking, current-model round trip, collection routing, and source-generated deserialization passed using System.Text.Json 8.0.0.0 in the default load context. A pvsite configuration containing scalars, a collection, and enum values round-tripped before and after loading the library. The control consumer requiring JSON 10 failed to bind. This is a host smoke test, not a full Civil 3D UI or grading regression test.

Other packages are unaffected: they do not reference the migration library, and this change does not edit their targets, workflows, central dependency versions, or shared build settings.

[Copilot]

Copilot review overview

🟡 Changes recommended

The .NET 8 resolver test no longer exercises its named cycle scenario, and compatibility documentation remains contradictory.

Review effort: Balanced
Findings: 1 · 2

Open (3)

• Separate .NET 8 snapshot and cycle-termination test scenarios · New
• Update documentation to reflect all three supported targets · New
• Include JsonTypeInfo.ElementType in targeted internals · New

What changed in this PR

Adds a .NET 8 package asset without requiring a newer System.Text.Json, while retaining .NET 10/11 behavior.

Changes:

• Multi-targets library, tests, samples, and benchmarks for .NET 8.
• Accesses internal .NET 8 collection metadata for routing compatibility.
• Expands routing tests, CI, analyzer, trimming, and NativeAOT validation.

File	Description
.github/​workflows/​egil-systemtextjson-migration-ci.yml	Adds .NET 8 CI coverage.
Egil.SystemTextJson.Migration/​AGENTS.md	Documents supported frameworks.
Egil.SystemTextJson.Migration/​README.md	Announces .NET 8 support.
Egil.SystemTextJson.Migration/​docs/​recipes/​aot-source-gen.md	Documents the .NET 8 resolver workaround.
Egil.SystemTextJson.Migration/​perf/​Egil.SystemTextJson.Migration.PerfTests/​Egil.SystemTextJson.Migration.PerfTests.csproj	Adds the .NET 8 benchmark target.
Egil.SystemTextJson.Migration/​samples/​Egil.SystemTextJson.Migration.Samples/​Egil.SystemTextJson.Migration.Samples.csproj	Adds the .NET 8 sample target.
Egil.SystemTextJson.Migration/​scripts/​verify-analyzer-package.ps1	Parameterizes analyzer verification by framework.
Egil.SystemTextJson.Migration/​scripts/​verify-trimming-package.ps1	Verifies all three framework assets.
Egil.SystemTextJson.Migration/​src/​Egil.SystemTextJson.Migration/​Egil.SystemTextJson.Migration.csproj	Adds the .NET 8 package asset.
Egil.SystemTextJson.Migration/​src/​Egil.SystemTextJson.Migration/​Migrations/​SourceValueShape.cs	Uses the compatibility accessor on .NET 8.
Egil.SystemTextJson.Migration/​src/​Egil.SystemTextJson.Migration/​Migrations/​StjInternals.cs	Adds access to internal collection metadata.
Egil.SystemTextJson.Migration/​test/​Egil.SystemTextJson.Migration.Analyzers.Tests/​Egil.SystemTextJson.Migration.Analyzers.Tests.csproj	Runs analyzer tests on .NET 8.
Egil.SystemTextJson.Migration/​test/​Egil.SystemTextJson.Migration.Tests/​CollectionContractCompatibilityTests.cs	Adds collection-routing compatibility coverage.
Egil.SystemTextJson.Migration/​test/​Egil.SystemTextJson.Migration.Tests/​Egil.SystemTextJson.Migration.Tests.csproj	Runs runtime tests on .NET 8.
Egil.SystemTextJson.Migration/​test/​Egil.SystemTextJson.Migration.Tests/​ResolverChainTests.cs	Adapts resolver setup for .NET 8.

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[egil]

One CI concern: out-of-support warnings for net8.0/net9.0 after .NET 11 GA. Suggested fix inline. The library changes look correct.

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Generated by Claude Code

[egil]

CI will likely break when .NET 8 and 9 go out of support (2026-11-10, same day as .NET 11 GA).

• After the SDK pin moves to 11.0.x (per the workflow comment), global.json rolls forward to that SDK.
• It will likely emit NETSDK1138 (target framework out of support) for net8.0 and net9.0.
• This loop builds the consumers with -warnaserror, and Release builds set TreatWarningsAsErrors in the root Directory.Build.props, so that warning becomes an error. SDK 9.0.100 did the same to net6.0 on its release day.

Suggested fix: opt out of the check with the documented CheckEolTargetFramework property. It states intent better than adding NETSDK1138 to NoWarn.

1. Library/test/perf/sample projects: add it to the root Directory.Build.props (or to a package-level one, if you'd rather keep it scoped to this package):

   <PropertyGroup Label="Compile settings">
     <!-- Deliberately keep targeting out-of-support TFMs (net8.0, net9.0). -->
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
   </PropertyGroup>

2. Generated consumer projects: these don't pick up the repo props. verify-analyzer-package.ps1 writes them to a temp dir, and verify-trimming-package.ps1 stubs out Directory.Build.props. So add the property inline to each generated <PropertyGroup>: Consumer.csproj, InvalidConsumer.csproj and SeverityConsumer.csproj in verify-analyzer-package.ps1, plus the trimming consumer in verify-trimming-package.ps1:

   <CheckEolTargetFramework>false</CheckEolTargetFramework>

   (Alternative: pass -p:CheckEolTargetFramework=false to each dotnet restore/build/publish call in the scripts.)

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Generated by Claude Code

[LiteracyFanatic]

Added CheckEolTargetFramework=false in package-level Directory.Build.props, which explicitly imports the root props so existing build settings remain intact. It is also inline in all three analyzer consumer projects and the trimming/AOT consumer project. Other packages retain their existing SDK support checks.

The Release solution build passed on all four targets with zero warnings/errors. I simulated SDK end-of-support checks for .NET 8/9 across the package projects and all four generated consumer templates: the new settings passed, while controls with CheckEolTargetFramework=true failed with NETSDK1138.

Updated in 87b1f31; the PR still contains one commit.