Let platform manifests go only on a confirmed collect, and copy again what a sweep took

An unrecorded index or a copy in progress can name a platform the records do not see, so
only a person's collect, after its dry run, takes an index's platforms, and only once every
kept index of each repository it touches was read. A copy missing a platform is copied
again, and a copy a build holds again is no longer recorded as collected (review of #144).

internal/inventory/builds.go

@@ -131,6 +131,11 @@ func (i *Inventory) RecordBuild(ctx context.Context, b Build) error {
 
 // RecordMirrored keeps that the mesh copied these bases into the artifact store (novox/hq ADR 0257):
 // by the build that said so, or by a person who says why. The first record of a copy stands.
+//
+// **A copy a build says it holds is held again.** The builder answers a copy only once the store holds
+// it whole, copying again what was let go of; so a copy the records say was collected, and a build now
+// says it copied, is no longer collected. Left collected, it would stay in the store and out of every
+// sweep for ever.
 func (i *Inventory) RecordMirrored(ctx context.Context, build, why string, references []string) error {
 	var by *string
 	if build != "" {
@@ -145,6 +150,12 @@ func (i *Inventory) RecordMirrored(ctx context.Context, build, why string, refer
 			 on conflict (reference) do nothing`, reference, by, why); err != nil {
 			return err
 		}
+		if build != "" {
+			if _, err := i.store.Pool().Exec(ctx,
+				`delete from artifact_collected where reference = $1`, reference); err != nil {
+				return err
+			}
+		}
 	}
 	return nil
 }