Take no dry run in: mark it on the request, echo it on the outcome, set it aside

A dry run of an unreviewed branch was heard by the daemon like any build, registered, and its
definition reached a machine (novox/hq issue 240). The mark now travels with the build and the
daemon records, registers and plans nothing for it.

internal/link/build.go

@@ -83,6 +83,10 @@ type BuildRequest struct {
 	// asking and echoed in the outcome, so whoever hears the outcome can register the module with
 	// its true source, whether or not they were the one who asked (novox/hq issue 176).
 	Source *SourceOnSeat `json:"source,omitempty"`
+	// DryRun says the asker wants the outcome to look at and nothing else (novox/hq issue 240): the
+	// builder echoes it, and whoever hears the outcome takes nothing in — no record, no registration,
+	// no plan, nothing a push could send.
+	DryRun bool `json:"dry-run,omitempty"`
 }
 
 // SourceOnSeat names a repository by the seat whose holder serves it and its path there.
@@ -144,6 +148,9 @@ type BuildResult struct {
 
 	// Source is the request's, echoed: the seat form of the repository, for whoever registers.
 	Source *SourceOnSeat `json:"source,omitempty"`
+
+	// DryRun is the request's, echoed: an outcome nobody may take in (novox/hq issue 240).
+	DryRun bool `json:"dry-run,omitempty"`
 }
 
 // ReadRepository is a repository a build read source from besides the module's own, at the branch,