Make RPC payload classification declaration-derived

Parent

Broad payload Schemas accept valid inputs and encoders while actual payloadless calls remain concise.

Classify payloadlessness from the actual declaration rather than void assignability.
Public API compiler fixtures cover Unknown, Any, void-containing unions, and true payloadless RPCs across key, query, mutation, infinite, accumulated, live, and encoder APIs.
Verify valid input-bearing calls at runtime and preserve default construction semantics.
Retain canonical JSON restrictions: broad input support does not imply that every broad-schema value is a valid default key.
Run the repository compiler and supported TypeScript 5.9 fixtures against workspace dependencies and update minimal contract guidance needed for the changed signatures.