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/prune_count_test.go

@@ -42,7 +42,7 @@ func TestTableCountForReportSurfacesReadFailure(t *testing.T) {
 	assert.Nil(t, missing, "a failed read is unknown, not a count")
 
 	// The rendered count for a failed read must say unknown, never 0.
-	assert.Equal(t, countUnknown, countText(missing))
+	assert.Equal(t, unknownText, countText(missing))
 	assert.NotEqual(t, "0", countText(missing))
 }
 
@@ -57,7 +57,7 @@ func TestCountTextDistinguishesEmptyFromUnknown(t *testing.T) {
 
 	assert.Equal(t, "0", countText(&zero))
 	assert.Equal(t, "7", countText(&seven))
-	assert.Equal(t, countUnknown, countText(nil))
+	assert.Equal(t, unknownText, countText(nil))
 }
 
 // TestCountDiffUnknownWhenEitherSideUnknown checks that a delta computed