Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

internal/link/events.go

@@ -290,6 +290,9 @@ type Checked struct {
 	Trunk  string            `json:"trunk,omitempty"`
 	Beside map[string]string `json:"beside,omitempty"`
 	Took   string            `json:"took,omitempty"`
+	// Built are the builds the check made for its merge, each with its reuse key and digests, or why not
+	// (novox/hq ADR 0350 §1): what the walk of the merge takes instead of building.
+	Built []CheckBuilt `json:"built,omitempty"`
 }
 
 // CheckedMember is one head a group's composed check judged, and the trunk commit it was merged into (novox/hq