[e128]

Executive Summary

Adds three union-aware design rules to the E128.Analyzers package, written against the typed UnionDeclarationSyntax. The typed node sits behind RSEXPERIMENTAL006 at the pinned Roslyn 5.9.0 band; the maintainer approved that suppression, which both component csproj files now carry.

• E128219 Memberless internal abstract base whose derivation stays in one assembly: the shape a closed hierarchy replaces. Code fix replaces the abstract modifier with the closed keyword and keeps trivia. Skips a base that already carries closed (closed alone implies abstract, per CS0144 probe).
• E128220 Union switch ending in a discard or var arm instead of a null arm. Code fix replaces a discard pattern with a null constant pattern; a var arm binds a value, so the fixer skips it. Probe finding: case _ on a union governing type parses _ as a case-type identifier (CS0103), so a statement catch-all is case var _:.
• E128221 Union Value read or cast outside a pattern match. No code fix, because the conversion changes control flow.

Shared helper UnionTypes.IsUnion accepts a source union through UnionDeclarationSyntax or a binary/hand-written union through the compiler-synthesized IUnion interface, matched by name and namespace chain because no metadata lookup resolves it.

Code Changes

• src/E128.Analyzers/Design/: three new analyzers plus UnionTypes.cs
• src/E128.Analyzers.CodeFixers/Design/: two new code fix providers
• Release files: rows promoted to AnalyzerReleases.Shipped.md under 2.10.0, PublicAPI.Unshipped.txt entries, README rule rows and sections
• csproj: RSEXPERIMENTAL006 NoWarn additions, version 2.9.8 to 2.10.0, README install snippet updated

Tests and Tooling

• 24 new tests across five files, plus UnionTestRuntime, a source stub for UnionAttribute, IsClosedTypeAttribute, and IUnion, because the net10.0 reference assemblies predate the union runtime. Fixture settings: LanguageVersion.Preview and an RSEXPERIMENTAL006 suppression.
• Full gate (scripts/check.sh) green: format, build, and the full suite, 2928 passed with 2 pre-existing skips. Test gate ran on the union sources before the commit; the docs-only and release-file follow-up edits did not rerun the suite.
• analyzer-release-check, readme-table-diff --analyzer, and readme-audit all clean after promotion.

Review Guide

Start with UnionTypes.cs for the detection contract, then each analyzer. The two-pass compilation-end shape of E128219 mirrors SealedByDefaultAnalyzer (E128005). Code fix tests for the compilation-end rule set SkipLocalDiagnosticCheck and NumberOfFixAllIterations = 1, the shape lode/dotnet/analyzers/testing.md records.

Rollback

Revert this single commit; the change set is self-contained to the analyzer package, its tests, and the release/README files.

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