The Kraken-Breaches of Ebonquay Seawall

The harbour town of Ebonquay is guarded by a long seawall, and every night the warden-mages of the Tidebound Circle raise their wards along it. Each ward covers one stretch of the wall. The Kraken of the Sallow Deep circles below, feeling for any stretch the wards leave bare. The Circle pays a bounty to whoever can list every breach before the tide turns.

Write findBreaches(length, wards).

• length is a positive integer: the wall runs from pace 0 to pace length, measured from the north tower.
• wards is a list of pairs [start, end] of integers with 0 <= start < end <= length. Each pair means the ward covers the wall from pace start to pace end. The list may be in any order. Wards may overlap, nest inside one another, or be exact duplicates. The list may be empty.

A breach is a maximal stretch [a, b] of the wall, with a < b, that lies within [0, length] and is not covered by any ward. Two wards that touch, with one ending at the same pace where another begins, leave no breach between them.

Return a list of every breach as [a, b] pairs of integers, sorted by a in increasing order. If the whole wall is covered, return an empty list.