Alternative runtime propagation contracts

The experimental RuntimePropagationContract compilation option selects a carrier protocol by assembly simple name and fully qualified interface metadata name. Workspace builds and the language server read the same project properties:

<RavenPropagationAssemblyName>NeoCLR.CoreProbe</RavenPropagationAssemblyName>
<RavenPropagationInterfaceType>System.Propagatable`3</RavenPropagationInterfaceType>

With this option, postfix ? requires exactly one matching self/output/residual interface. Public nongeneric TryGetOutput(out output) and TryGetResidual(out residual) return Boolean; a public static nongeneric FromResidual(residual) returns the carrier. The compiler checks the self argument and compatible residuals. Missing or partial configuration does not silently fall back to union naming conventions.

Selected-target propagation handles carrier results only. It does not synthesize exception capture around the operand. Faults remain failures; they are not converted into residuals. This differs from Raven's default .NET propagation lowering, which can capture compatible exceptions. Explicit try syntax remains separate and is not made portable by selecting this contract. A target importer must reject unsupported exception handling and validate the admitted metadata/operations.

With no descriptor, existing .NET interface discovery, union conventions and exception behavior are unchanged. Optional copies and incremental semantic reuse account for the selection. There is no new language syntax. This is an experimental target hook, not an automatic retargeting of arbitrary .NET programs.

The neoCLR bridge demonstrates Result<int, OverflowError>, Optional<int> and Result<Void, OverflowError> success and early residual return. Discarding a selected Void-output propagation does not leave storage on the evaluation stack. The installed 0.1.12-neoclr.3 tools predate this feature; use a source build of the experimental branch. Void as a type denotes a unit value; ordinary void-returning calls retain the CLR no-result convention. The target bridge projects value-storage signatures separately.