Read a shrink against the item's own path, so a moved directory starts its size history again (hq issue 368)

A reading now keeps the path it was measured at, and the peak is read only from
readings at the item's path now. Before, an agent's home that moved to its own
account was compared with the operator's home it left, and data-shrank was
raised for data that was never lost. Existing readings take their item's path
now, so a shrink that is real stays raised.

cmd/mesh-controller/data_test.go

@@ -169,6 +169,44 @@ func TestAShrinkOfMoreThanHalfIsUrgent(t *testing.T) {
 	}
 }
 
+// THE FALSE ALARM (issue 368), replayed through the store D13 reads: an agent's home moved from the
+// operator's own home (94.7 MB) to the agent account's fresh one (490 B), and `data-shrank` was raised
+// for data that was never lost. A moved item is read against its new path only, so nothing is raised —
+// and a genuine shrink at the new path, a week of history later, still is.
+func TestAMovedPathIsNoShrinkAndAShrinkThereStillIs(t *testing.T) {
+	inv := inventory.ForTest(t)
+	ctx := t.Context()
+	shelf := shelfFor(t, houseManifest)
+	declared := []inventory.DeclaredData{{Module: "house", Item: "config", Class: "irreplaceable", Owned: true}}
+	start := time.Now().Add(-6 * time.Hour)
+	measure := func(at time.Time, path string, size int64) []conditions.Observation {
+		t.Helper()
+		if _, err := inv.RecordData(ctx, "home", declared, map[string]map[string]inventory.Measurement{"house": {
+			"config": {Path: path, Size: bytesOf(size), MeasuredAt: when(at), LastWrite: when(at),
+				LastBackup: when(at)}}}, "", at); err != nil {
+			t.Fatal(err)
+		}
+		records, err := inv.Data(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		peaks, err := inv.DataPeaks(ctx, at.Add(-shrinkWindow))
+		if err != nil {
+			t.Fatal(err)
+		}
+		return dataFindings(records, peaks, shelf, nil, nil, at)
+	}
+	measure(start, "/home/operator/.claude", 94_700_000)
+	if got := findingsByKind(measure(start.Add(10*time.Minute), "/home/agent/.claude", 490)); got[kindDataShrank].Kind != "" {
+		t.Fatalf("a moved path raised a shrink: %+v", got[kindDataShrank])
+	}
+	measure(start.Add(2*time.Hour), "/home/agent/.claude", 300<<20)
+	got := findingsByKind(measure(start.Add(4*time.Hour), "/home/agent/.claude", 1<<20))[kindDataShrank]
+	if got.Severity != conditions.Urgent || !strings.Contains(got.Summary, "shrank") {
+		t.Fatalf("a genuine shrink at the new path was not raised: %+v", got)
+	}
+}
+
 // Data said to be written all the time and not written; data with no backup or an old one — urgent when
 // irreplaceable, a warning when valuable; and a new item given its bound before it is said.
 func TestQuietDataAndMissingBackupsAreSaidByClass(t *testing.T) {