fix: validate interval bounds in Union before appending

The Union function computes intersection intervals by taking max(start)
and min(end) of two intervals. However, when intervals don't actually
overlap (e.g., [90,95] and [80,88]), this produces invalid intervals
where start > end (e.g., [90,88]).

This causes a panic in TotalWhitespace when it tries to slice with
invalid bounds: `lines[90:88]` -> "slice bounds out of range [90:88]"

The fix validates that start <= end before appending to the result,
which correctly handles non-overlapping intervals by excluding them.

Added test cases for non-overlapping and adjacent intervals.

Author: bjaus

interval/interval.go

@@ -98,10 +98,13 @@ func Union(a []Interval, b []Interval) []Interval {
 			i++
 		}
 
-		result = append(result, Interval{
-			Start: start,
-			End:   end,
-		})
+		// Only append if the intervals actually overlap (start <= end)
+		if start <= end {
+			result = append(result, Interval{
+				Start: start,
+				End:   end,
+			})
+		}
 	}
 
 	return joinSortedIntervals(result)

interval/interval_test.go

@@ -77,6 +77,28 @@ func TestUnion(t *testing.T) {
 				{Start: 5, End: 5},
 			},
 		},
+		{
+			name: "non-overlapping intervals should produce empty result",
+			a: []interval.Interval{
+				{Start: 90, End: 95},
+			},
+			b: []interval.Interval{
+				{Start: 80, End: 88},
+			},
+			e: []interval.Interval{},
+		},
+		{
+			name: "adjacent but non-overlapping intervals",
+			a: []interval.Interval{
+				{Start: 10, End: 20},
+				{Start: 50, End: 60},
+			},
+			b: []interval.Interval{
+				{Start: 21, End: 30},
+				{Start: 40, End: 49},
+			},
+			e: []interval.Interval{},
+		},
 	}
 
 	for _, tc := range testacses {