Count a platform manifest as its index's, and list the unrecorded copies of upstream images

The store's tools counted each platform of an upstream image apart from its index, so a
kept copy read as unrecorded and a let-go index's bytes were never counted as freed. The
copies no record names are now listed for the controller's mirrors verb, and a dry run
says what stays yet names a platform collection would take (hq ADR 0257, issue 321).

modules/artifact-store-tools/README.md

@@ -50,6 +50,18 @@ it lists. The store's collector removes every blob no manifest marks, so these n
 | `named-document` | a one-layer manifest the controller keeps under a tag (hq to-be 45 §9) | no |
 | `unrecorded` | no record names it | **never**, by any tool (ADR 0189 §3) |
 
+**A platform manifest takes the state of the recorded index naming it** (hq ADR 0257). An upstream
+image is an index over several platforms, and each platform manifest is a manifest of the repository in
+its own right; the controller lets an index go with the platform manifests only it names, so they are
+counted as the index's. Two indexes naming one platform give it the more keeping state. An index no
+record names leaves its platform manifests unrecorded with it.
+
+**Unrecorded copies of upstream images** are listed in `unrecorded_mirrors`: each manifest at the top of
+a repository only the build's mirror writes (`upstream/<host>/<path>`, or a module's `<module>/on-<argument>`
+from before hq ADR 0257) that no record names, with what it and its platforms mark alone. They are the
+input to the controller's `mirrors` verb, which records them as the mesh's on a person's word; the sweep
+then decides about them as about every other copy. This tool removes none of them.
+
 The records come from the controller's `artifacts` verb, asked with the references already let go of;
 when that answer is too large to carry, it is asked without them and the answer says so.
 
@@ -60,6 +72,11 @@ lets go of (ADR 0189 §2), so a real run asks its `collect` with `why` and `conf
 every kept archive first and records the run as a hand-act. Bytes come back at the store's nightly
 collection, which runs with the store held still.
 
+An index goes with the platform manifests it names that no kept index of its repository also names, as
+the controller's sweep lets them go (hq ADR 0257). The controller reads only its records; the store's
+files show what else names a platform, so a dry run lists in `platforms_named_by_what_stays` every
+manifest that stays and would lose a platform it names — to be read before a real run.
+
 ## Tests
 
 ```