Interface ClashRevisionDiff

The result of comparing a previous clash run to a later ("next") one.

• added clashes present in next but not in previous (new issues)
• persistent clashes present in both runs (still open; the next Clash)
• resolved clashes present in previous but not in next (fixed/removed)

"Present in" counts occurrences, not just review keys: two clashes sharing a review key in one run need a counterpart each to both be persistent.

Each array is sorted by clash.id for deterministic, diff-friendly output.