Use two-tip diffs and audit fleet instances separately

When main and staging contain independent promotions or cherry-picks, use the two-tip diff for what is actually new; a three-dot merge-base diff can reclassify already promoted changes. Use cherry equivalence only as supporting history. Audit deployment variables per fleet instance, not only the template, because sync may retain instance-only flags and create an implicit one-instance canary. Make that asymmetry an explicit release decision and validate its dependency path.