Add Junctions() API for simplified train definitions

[Theauxm]

Summary

• Add Junctions() virtual method on Train<TInput, TReturn> that returns TReturn directly, replacing the Activate(input).Chain<T>().Resolve() ceremony
• Add implicit conversion operator on Monad<TInput, TReturn> so the chain result flows into the return type without explicit Resolve()
• Expose Chain/Extract/ShortCircuit/AddServices/IChain as protected methods on Train that delegate to the internal Monad
• RunInternal becomes virtual (was abstract) with a default implementation that calls Junctions() — fully backwards compatible

Before:

protected override async Task<Either<Exception, int>> RunInternal(string input) =>
    Activate(input).Chain<StringLengthJunction>().Resolve();

After:

protected override int Junctions() =>
    Chain<StringLengthJunction>();

Test plan

• 12 new unit tests covering happy path, failure path, Extract, implicit conversion, and backwards compat
• All 208 existing Trax.Core tests pass
• Zero build warnings

[traxsharp]

This PR is included in version 1.4.0