Design explicit multi-outcome command responses

[woksin]

Arc for .NET classifies OneOf and Result response unions with several visible members. Arc for TypeScript supports a single visible response type and rejects ambiguous unhandled alternatives at generation. Design an explicit, typed way for a command to return one of several success shapes (plus rejected/denied), with a stable wire contract matching .NET and generated client types, without changing the default wire behavior.

[woksin]

Design finding (source inspection of Arc for .NET v22.23.0 and main at 1b5df0b6, the .NET proxy generator, and the @cratis/arc client):

• .NET unwraps the active OneOf/Result value into response with no branch index or discriminator. An arbitrary error DTO is not automatically a failure; only recognized validation/authorization values become envelope failures.
• The .NET proxy generator emits one response type per command: it drops server-handled alternatives and picks a concept, primitive, or the first remaining candidate. The client hydrates every response with that single constructor, so different success shapes are not distinguishable on the client.
• Arc for TypeScript already matches the wire behavior for a single visible response and rejects ambiguous alternatives at generation instead of choosing one silently.

Plan: bounded parity. (1) Add paired conformance cases characterizing .NET for OneOf/Result with DTO, primitive, validation, denial, arbitrary error DTO and tuple alternatives. (2) Share branch analysis between the proxy generator and runtime metadata so alternatives that resolve to the same client representation are accepted. (3) Document that several business outcomes should use one response DTO with an application-owned status field. True client-visible unions would need coordinated .NET generator and client changes with an application-owned discriminant; no implicit wire discriminator will be invented.

[woksin]

Bounded parity released in v0.39.0 (#91). True client-visible response unions would need coordinated Arc for .NET generator and client changes.