Mark the range unions closed (C# 15) for compile-time exhaustiveness, on both lines #149

Decided 2026-10-18 (maintainer, after the #148 probe): closed, not union. Ships on both lines.

• closed on every union type (Int32Range … TimeRange, the three NodaTime ones, YearMonthRange) and on RangeSet where a shape switch exists. closed is implicitly abstract, so the abstract modifier goes. The private constructor stays: a closed type is only assembly-scoped (CS9382 outside the assembly).
• An in-project System.Runtime.CompilerServices.IsClosedTypeAttribute polyfill under #if !NET11_0_OR_GREATER in each shipping assembly; a net11.0 consumer reads it from metadata and gets exhaustiveness all the same (measured).
• Consumers keep working unchanged: an existing _ => throw arm next to complete arms raises nothing; a missing arm is CS8509 naming the variant. So this is not breaking for consumers, and the ApiCompat pack should stay clean (verify).
• Both lines. First on net11 as 11.0.0-preview.2. On 10.x it ships as a minor after .NET 11 GA, because a stable package must not build with LangVersion preview; from GA on, LangVersion 15 is stable and the 10.x line builds with the .NET 11 SDK targeting net10.0. CI on the 10 line then needs the .NET 11 SDK too (CI matrix for two lines: .NET 10 and 11 SDKs, PostgreSQL floor and 18 legs per line #152).
• union is not adopted: it changes the representation (boxed object? value, different default) and every consumer's declared type for no gain in exhaustiveness.

The headline of 11.0.0. Today the range union is a closed class hierarchy only at runtime (AGENTS.md, "Critical Rules"): a switch over Int32Range warns CS8509 without a discard, so consumers write _ => throw, the three engines throw ShapePair.Unreachable, and EngineDispatchConventionTests parses src/**/Internals/ to make sure the discards throw. The rewrite makes the compiler carry that guarantee.

Scope, per the probe's outcome:

• Every union type (Int32Range … TimeRange, the three NodaTime ones, YearMonthRange) and RangeSet where a shape switch exists.
• Consumers gain compile-time exhaustiveness: a switch over a union's variants loses its _ => throw. The three engines do not: they switch over the structural interface patterns (IFiniteRange<T>, …) on IRange<T> operands, and closing the concrete classes does not make those switches exhaustive. ShapePair.Unreachable and EngineDispatchConventionTests stay as they are. Whether a per-type closed dispatch inside the engines is worth it is decided in Compile probe: C# 15 closed hierarchies and union types for the five variants, on net10.0 and net11.0 #148, not promised here.
• JSON, TypeConverter and the EF variant retyper (RangeVariantConstantRetyper) are re-measured: an inlined constant still arrives typed as its variant.
• ApiCompat: generate the suppression file and document every entry in the changelog's "what can break you" list; docs/migration.md gets the v10.x section showing a consumer switch before and after.
• Native AOT: the consumer smoke test's AOT consumer must switch over a range.
• The private constructor stays if the probe shows closed is assembly-scoped only.

Not on the 10.x line unless the probe shows the feature compiles on net10.0 with no runtime support.