compact: reclaim superseded duplicates left after a full check --repair #10471

Follow-up to #10443 (review there).

When a crashed borg create is re-run and the archive is kept, the second run stores the lost chunks again, so the crashed run's packs hold only copies of chunks indexed elsewhere (superseded duplicates: unindexed objects whose chunk id the index maps to another location).

• A full borg check --repair rebuilds the index with one entry per chunk id (build_chunkindex_from_repo), so these copies stay unindexed.
• compact_packs rewrites a pack only if indexed - used is above the threshold. For these packs it is 0, so they are never rewritten and compact_pack never drops the duplicates.
• Every borg compact logs the unindexed-bytes hint again, and the space is never reclaimed.

Repro: kill -9 a borg create of a 1.5 GB random file after 3s, re-run the same create, keep the archive. 1.07 GB stays unindexed and the packs stay at 2.5G through two full borg check --repair + borg compact runs. borg check is rc 0.

Proposal: classify unindexed objects in compact_packs pass 1 as well (as compact_pack does via superseded_gap_ranges), so superseded bytes count toward the rewrite threshold and a pack holding only superseded duplicates can be deleted. See the TODO(#9868 follow-up) in compact_pack and the TODO: in compact_packs.