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/rank.go

@@ -208,14 +208,7 @@ const SpokenLimit = 5
 // DayPlan.Spoken is built core-side: two formatters drift, and then she says
 // one order and shows another.
 func FormatRU(ranked []Ranked) string {
-	var open, cands []Ranked
-	for _, r := range ranked {
-		if r.Status == StatusCandidate {
-			cands = append(cands, r)
-		} else {
-			open = append(open, r)
-		}
-	}
+	open, cands := split(ranked)
 	if len(open) == 0 && len(cands) == 0 {
 		return say.S(say.TasksNone, nil)
 	}
@@ -243,6 +236,20 @@ func FormatRU(ranked []Ranked) string {
 	return b.String()
 }
 
+// split separates confirmed work from candidates, preserving Rank's order
+// within each half. FormatRU and Spoken have to agree on where the line falls,
+// so they read it from one place.
+func split(ranked []Ranked) (open, cands []Ranked) {
+	for _, r := range ranked {
+		if r.Status == StatusCandidate {
+			cands = append(cands, r)
+		} else {
+			open = append(open, r)
+		}
+	}
+	return open, cands
+}
+
 // joinRU lists up to limit tasks, then says how many are left. withReasons
 // attaches the parenthesised reason — candidates are listed bare, since their
 // due dates are Maven's reading of a mail and not something he stated.
@@ -273,14 +280,7 @@ func joinRU(rs []Ranked, limit int, withReasons bool) string {
 // an ordinal resolves against is built here and not by a caller guessing how
 // the renderer split and truncated it.
 func Spoken(ranked []Ranked) []Ranked {
-	var open, cands []Ranked
-	for _, r := range ranked {
-		if r.Status == StatusCandidate {
-			cands = append(cands, r)
-		} else {
-			open = append(open, r)
-		}
-	}
+	open, cands := split(ranked)
 	out := make([]Ranked, 0, 2*SpokenLimit)
 	for _, group := range [][]Ranked{open, cands} {
 		if len(group) > SpokenLimit {