Minor updates to overlap detection and segment coalescing logic

internal/coalesce/coalesce_test.go

@@ -115,6 +115,51 @@ func TestApplyHonorsCurrentOrder(t *testing.T) {
 	}
 }
 
+func TestApplyUsesEffectiveRunEndForReorderedSegments(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 10, 20, "long"),
+			segment("a.json", 1, "Alice", 1, 2, "early"),
+			segment("a.json", 2, "Alice", 22, 23, "after long"),
+		},
+	}
+
+	got, summary := Apply(merged, 3)
+	if summary.OriginalSegmentsMerged != 3 || summary.CoalescedSegments != 1 {
+		t.Fatalf("summary = %#v", summary)
+	}
+	if len(got.Segments) != 1 {
+		t.Fatalf("segment count = %d, want 1", len(got.Segments))
+	}
+	if got.Segments[0].Text != "long early after long" {
+		t.Fatalf("text = %q", got.Segments[0].Text)
+	}
+	if got.Segments[0].Start != 1 || got.Segments[0].End != 23 {
+		t.Fatalf("bounds = %f-%f, want 1-23", got.Segments[0].Start, got.Segments[0].End)
+	}
+}
+
+func TestApplyDoesNotMergeBeyondEffectiveRunEndGap(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 10, 20, "long"),
+			segment("a.json", 1, "Alice", 1, 2, "early"),
+			segment("a.json", 2, "Alice", 23.1, 24, "too late"),
+		},
+	}
+
+	got, summary := Apply(merged, 3)
+	if summary.OriginalSegmentsMerged != 2 || summary.CoalescedSegments != 1 {
+		t.Fatalf("summary = %#v", summary)
+	}
+	if len(got.Segments) != 2 {
+		t.Fatalf("segment count = %d, want 2", len(got.Segments))
+	}
+	if got.Segments[0].Text != "long early" || got.Segments[1].Text != "too late" {
+		t.Fatalf("segments = %#v", got.Segments)
+	}
+}
+
 func TestApplyDerivedProvenanceForMixedSourcesAndDerivedInputs(t *testing.T) {
 	first := segment("a.json", 0, "Alice", 1, 2, "first")
 	second := model.Segment{