tasks: count overdue by calendar day like the ranker does (V-581)

Stalls compared the due instant to now while Rank compares whole calendar days, so a task due at 09:00 was counted overdue from 09:01 while its own row on the same page still read the reason as today. Stalls now reads dayDelta.

A row with no capture time also counted as sitting, because the zero time is January of year 1 and every span from it clears ten days. Rank already guarded that and Stalls did not.

Folded the open-versus-candidate partition FormatRU and Spoken each carried into one split helper. The two have to agree on where that line falls.

internal/tasks/stall_test.go

@@ -54,6 +54,27 @@ func TestStallsSaysNothingWhenThereIsNothing(t *testing.T) {
 	}
 }
 
+func TestStallsCountsOverdueByCalendarDay(t *testing.T) {
+	// Same reading as Rank, or the count above the table contradicts the reason
+	// in the row: due at 09:00, asked at 12:00, and it is still due today.
+	now := stallNow()
+	earlier := now.Add(-3 * time.Hour)
+	items := []Item{{ID: 1, Text: "оплатить интернет", Status: StatusOpen, Created: now.Add(-time.Hour), Due: &earlier}}
+	if got := Stalls(items, now); len(got) != 0 {
+		t.Fatalf("shapes = %+v, want none — a task due today is not overdue", got)
+	}
+}
+
+func TestStallsIgnoresATaskWithNoCaptureTime(t *testing.T) {
+	// The zero time is January of year 1, so an unstamped row would count as
+	// sitting forever.
+	now := stallNow()
+	items := []Item{{ID: 1, Text: "купить молоко", Status: StatusOpen}}
+	if got := Stalls(items, now); len(got) != 0 {
+		t.Fatalf("shapes = %+v, want none", got)
+	}
+}
+
 func TestStallsCountsNoJudgement(t *testing.T) {
 	// The line this shape may not cross. Every sentence states a count; none of
 	// them says whether the work matters or should be dropped.