A growth-proof staged builder over the generated ExecutionInput factory

The generated Graphitron.newOwnedExecutionInput(...) (and its escape-hatch sibling) takes one positional parameter per contextArgument, so its arity grows with the number of declared contextArguments plus the <sessionState> mount’s payload parameters, and every addition is a source-incompatible signature change at the consumer’s call sites. A consumer can bundle payload parameters into one carrier type today, but nothing gives the factory a growth-proof named surface. A staged builder over the factory (one named, typed step per contextArgument slot, compile-checked for completeness) would apply to contextArguments generally. Named as a separate concern in the session-identity work’s trade-offs; filed here so it survives that spec’s deletion at Done.