Measure each item its own way; never compare a partial size (hq ADR 0233)

A walk over a large library every hour loads the array that protects it. An item now says how it
is measured — a bounded daily walk, a dataset's counters, or its top level only — and a size that
is a lower bound is kept as such and never read as a shrink.

internal/inventory/data_test.go

@@ -107,3 +107,24 @@ func TestReadingsAreHourlyAndThePeakIsTheLargest(t *testing.T) {
 		t.Fatalf("the newest measurement is not the one on record: %+v", r)
 	}
 }
+
+// A partial walk's lower bound is kept as the newest measurement, said as partial, and never kept as a
+// reading a shrink would be read against.
+func TestAPartialMeasurementIsNeverAReading(t *testing.T) {
+	inv := fresh(t)
+	ctx := t.Context()
+	now := time.Date(2026, 10, 6, 0, 0, 0, 0, time.UTC)
+	declared := []DeclaredData{{Module: "big", Item: "data", Class: "valuable", Owned: true}}
+	if _, err := inv.RecordData(ctx, "home", declared, map[string]map[string]Measurement{"big": {"data": {
+		Path: "/srv/big", Size: size(5), MeasuredAt: at(now), Precision: "partial: stopped"}}}, "", now); err != nil {
+		t.Fatal(err)
+	}
+	var n int
+	if err := inv.store.Pool().QueryRow(ctx, `select count(*) from data_reading`).Scan(&n); err != nil || n != 0 {
+		t.Fatalf("%d readings from a partial measurement: %v", n, err)
+	}
+	r, err := inv.DataOf(ctx, "home", "big", "data")
+	if err != nil || r.Precision != "partial: stopped" || Comparable(r.Precision) {
+		t.Fatalf("%+v, %v", r, err)
+	}
+}