Show one row per recurring chore in the flat task list

The recurrence scheduler materializes a series' next instance once the
current one comes due, regardless of whether it was completed -- that's
deliberate, so ignoring an occurrence doesn't stall the series. The side
effect is that the flat task list shows an unfinished chore and its
already-generated successor at the same time: "Bills" appears twice right
now (Sep 1 overdue + Oct 1), as does "Bones's meds".

CollapseRecurringSeries keeps only the earliest-due open instance per
series, which is the one actually owed. Applied to the atoms list only --
the timeline and widget are date-anchored, where each instance correctly
belongs to its own day, so they keep all of them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

1 files changed, 40 insertions, 0 deletions

diff --git a/internal/models/recurrence_test.go b/internal/models/recurrence_test.go
index 8d18996..affba56 100644
--- a/internal/models/recurrence_test.go
+++ b/internal/models/recurrence_test.go
@@ -49,3 +49,43 @@ func TestComputeNextOccurrence(t *testing.T) {
 		})
 	}
 }
+
+func TestCollapseRecurringSeries_KeepsOnlyEarliestOpenInstance(t *testing.T) {
+	sep1 := time.Date(2026, 9, 1, 0, 0, 0, 0, time.UTC)
+	oct1 := time.Date(2026, 10, 1, 0, 0, 0, 0, time.UTC)
+
+	tasks := []Task{
+		{ID: "bills-sep", Content: "Bills", DueDate: &sep1, RecurrenceSeriesID: "bills"},
+		{ID: "bills-oct", Content: "Bills", DueDate: &oct1, RecurrenceSeriesID: "bills"},
+		{ID: "one-off", Content: "Fix curtain holder"},
+	}
+
+	got := CollapseRecurringSeries(tasks)
+
+	var ids []string
+	for _, task := range got {
+		ids = append(ids, task.ID)
+	}
+	if len(got) != 2 {
+		t.Fatalf("got %d tasks (%v), want 2: one row per recurring series plus the one-off", len(got), ids)
+	}
+	if got[0].ID != "bills-sep" {
+		t.Errorf("kept %q for the Bills series, want bills-sep (the earliest open instance is the one you actually owe)", got[0].ID)
+	}
+	if got[1].ID != "one-off" {
+		t.Errorf("non-recurring task was dropped or reordered: got %q", got[1].ID)
+	}
+}
+
+func TestCollapseRecurringSeries_UndatedAndDistinctSeriesUntouched(t *testing.T) {
+	due := time.Date(2026, 9, 1, 0, 0, 0, 0, time.UTC)
+	tasks := []Task{
+		{ID: "a", RecurrenceSeriesID: "series-a", DueDate: &due},
+		{ID: "b", RecurrenceSeriesID: "series-b", DueDate: &due},
+		{ID: "c", RecurrenceSeriesID: "series-c"}, // recurring but undated
+	}
+
+	if got := CollapseRecurringSeries(tasks); len(got) != 3 {
+		t.Errorf("len = %d, want 3 -- distinct series must not collapse into each other", len(got))
+	}
+}