Report unknown blob figures for a snapshot whose manifest cannot be read (closes #272)

When remote info could not read a snapshot's manifest, the orphan
figures were unknown but the snapshot's row still gave 0 blobs and 0 B,
in the table and in --json. The row's blob count and blob size are now
unknown, and null in --json.

A directory with no manifest, as an interrupted backup leaves, still
shows 0: the orphan figures count its blobs as orphaned, so it
references none.

The constant holding the "unknown" text is renamed from countUnknown to
unknownText, since it now also stands for a size.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -1756,9 +1756,9 @@ func (v *Vaultik) PruneDatabase() (*PruneResult, error) {
 	return result, nil
 }
 
-// countUnknown is what a count reads as when its query could not be run,
-// distinct from "0", which means the table really was empty.
-const countUnknown = "unknown"
+// unknownText is what a count or size reads as when it could not be
+// determined, distinct from "0", which is a real zero.
+const unknownText = "unknown"
 
 // tableCountForReport returns the row count of a table for the prune
 // summary, or nil if the count could not be read. A read failure is
@@ -1795,7 +1795,7 @@ func countDiff(before, after *int64) *int64 {
 // one that could not be queried.
 func countText(count *int64) string {
 	if count == nil {
-		return countUnknown
+		return unknownText
 	}
 
 	return strconv.FormatInt(*count, 10)