* perf(backend): cap cumulative diff bytes and file count during large rebases
* perf(web): coalesce cumulative-diff refetches and skip redundant status compares
* feat(web): warn when the changes panel hides files from a capped diff
* fix(backend): enforce cumulative diff total byte budget strictly
* fix(web): keep coalesced refetch timer shared across envKey subscribers
* fix(web): drop unsound git-status SHA fast-path that staled live diffs
* fix(web): cancel coalesced timer on mid-flight drain to avoid duplicate refetch
* refactor(web): move changes-panel banner and helper to a sibling file
* docs(backend): clarify cumulative diff truncated-files-count semantics
* fix(web): tighten useSelectedFileKey mode param to the all|file union
Henry