feat: async chain end to end

[Theauxm]

Summary

Make Junctions() return Task<Either<Exception, TReturn>> and the fluent Monad chain (Chain, Resolve, Extract, ShortCircuit, AddServices) compose asynchronously end to end. Method names are unchanged. The awaitable struct MonadTask<TInput, TReturn> wraps Task<Monad<,>> so .Chain<A>().Chain<B>().Resolve() keeps composing without forcing all generic type arguments at every link.

Eliminates the deadlock class that bit Blazor Server / WPF consumers under single-threaded sync contexts. The previous suppression hack in Monad.Chain was a band-aid that moved the deadlock around rather than fixing it; with the chain truly async, the bug class is structurally impossible.

What's deleted

• The implicit TReturn(Monad<,>) operator (C# forbids implicit Task<T> conversion)
• Internal Chain<TJunction, TIn, TOut>(... out Either<,>) overloads (out is illegal in async)
• The SynchronizationContext suppression block in Monad.Chain
• TrainSyncContextTests.cs — the deadlock class is structurally impossible now

Internal renames

Chain → ChainJunction, ShortCircuitChain → ShortCircuitJunction to free the public name space (so the public surface can offer Chain<TJ, TIn, TOut>(TJ junction) returning MonadTask<,> without colliding with the reflection-invoked async helper). ReflectionHelpers updated to match.

Why MonadTask exists

C# does not perform partial generic inference between explicit type arguments and the receiver type. A plain extension method on Task<Monad<,>> would force callers to write .Chain<JunctionA, MyInput, MyOutput>() at every continuation. Wrapping the Task in a struct that already knows TInput/TReturn at the type level lets callers stick with .Chain<JunctionA>().

Surprise: TrainChainAnalyzer needed no changes

The fluent member-access syntax tree the analyzer walks is unchanged — only the runtime types flowing through it differ. All 24 analyzer tests pass unmodified.

Test plan

• All four test projects pass (162 unit + 16 sanity + 16 integration + 24 analyzer = 218)
• dotnet build zero warnings
• dotnet csharpier check . passes
• Downstream repos build clean against the new package (covered by paired PRs)

Migration

See Trax.Docs migration guide PR for the consumer-facing migration walkthrough. Versioned as a minor — consumers pinned via Version="1.*" will pull this and need a one-line edit per train override before their projects compile.

[traxsharp]

This PR is included in version 1.5.0