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/inventory/plans.go

@@ -286,6 +286,9 @@ type PlanModule struct {
 	FirstAt *time.Time `json:"first_at,omitempty"`
 	Commit  string     `json:"commit,omitempty"`
 	Why     string     `json:"why,omitempty"`
+	// Made is what the walk's build of it did (novox/hq ADR 0350 §4): "reused from <check build>", naming the pull
+	// request's head, the trunk commit and the digests, or "built: <why>". Said in `plan-moved` and kept here.
+	Made string `json:"made,omitempty"`
 	// Build is the id of the build the plan asked for this module (novox/hq ADR 0219), so the plan
 	// matches its outcome by id — the one thing every outcome echoes, a failed one that never learnt
 	// its module's name included. Empty in a plan from before it was kept, which is matched by