Normalize zero outcomes and omitted diagnostic delivery state

outcome.go

@@ -49,6 +49,7 @@ type Diagnostic struct {
 // Cause records the operation error. DeliveryError records an error encountered
 // while presenting diagnostics or reports; it is kept separate from Cause so
 // both failures remain inspectable. Outcome does not implement error.
+// Invocation coordination treats a zero Outcome as success with status 0.
 type Outcome struct {
 	// Kind classifies the result.
 	Kind OutcomeKind