morning, routine: reject the two configs that silently do nothing (V-581)

Both Due functions key their last-fired map by routine name, so two routines sharing a name took turns suppressing each other and one of them never fired. Validate now rejects a duplicate name in either package.

parseHHMM checked the digits arithmetically, which let a stray character cancel out: window_start of 2 :00 loaded as 04:00 and passed the validation that exists to catch that typo. Each of the four positions is now checked as a digit, which makes the negative bounds unreachable and they are gone.

Folded the three copies of the unevidenced-item loop in Evaluate, Outstanding and Due into one helper.

internal/routine/routine.go

@@ -38,13 +38,22 @@ type Routine struct {
 }
 
 // Validate reports the first structural problem with a routine set: a missing
-// name/cron/body or an unparseable cron expression. Called at config load so a
-// typo surfaces at startup, not as a silently-never-firing routine at runtime.
+// name/cron/body, a duplicate name or an unparseable cron expression. Called at
+// config load so a typo surfaces at startup, not as a silently-never-firing
+// routine at runtime.
+//
+// Names must be unique because Due keys its last-fired map by name. Two
+// routines sharing one would take turns being suppressed by each other's fire.
 func Validate(routines []Routine) error {
+	seen := make(map[string]bool, len(routines))
 	for _, r := range routines {
 		if r.Name == "" {
 			return fmt.Errorf("routine: name is required")
 		}
+		if seen[r.Name] {
+			return fmt.Errorf("routine %q: duplicate name", r.Name)
+		}
+		seen[r.Name] = true
 		if r.Body == "" {
 			return fmt.Errorf("routine %q: body is required", r.Name)
 		}