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/catalogue/verbs.go

@@ -449,7 +449,9 @@ var ControllerVerbs = []Verb{
 	{Name: "collect", Description: "The artifact store's sweep, now: what the mesh made and keeps for no reason " +
 		"is let go of. Without confirm a dry run — every kept archive asked about with HEADs, what would be let go " +
 		"listed, nothing held or deleted. With confirm and why: every kept archive held first, then each eligible " +
-		"artifact let go of, oldest first, and recorded collected — a hand act, recorded with its why. Bounded by " +
+		"artifact let go of, oldest first, and recorded collected — a hand act, recorded with its why. A confirmed " +
+		"collect lets an index go with the platform manifests no kept index names (read first; any read that " +
+		"fails lets nothing go); the sweep after a build lets an index go alone (novox/hq ADR 0257). Bounded by " +
 		"most (500 by default, at most 5000) and 45 seconds; what is left is said. Bytes are reclaimed by the " +
 		"store's nightly collector. Never touches a digest the mesh did not record (novox/hq ADR 0189, ADR 0251).",
 		Input: schema(map[string]string{
@@ -463,7 +465,8 @@ var ControllerVerbs = []Verb{
 		"before copies were recorded, or by a build that failed before then) to record as the mesh's, so the " +
 		"sweep keeps or lets go of them like any other; only a repository the mirror writes is taken " +
 		"(upstream/<host>/<path>, or a module's <module>/on-<argument>), and each must be in the store. Without " +
-		"confirm a dry run; with confirm and why, recorded — a hand act. Records only: nothing is deleted " +
+		"confirm a dry run; with confirm and why, recorded — a hand act. A recorded copy is eligible: the next " +
+		"sweep lets it go unless a kept build stood on it. Records only: this verb deletes nothing " +
 		"(novox/hq ADR 0257).",
 		Input: schema(map[string]string{
 			"record":  "references to record, artifact-store://<repository>@sha256:<hex>, separated by spaces or commas",