Add an opt-in benchmark that compares fast-mode exact incremental indexing against a fresh fast full rebuild in an isolated temp repo and CBM_CACHE_DIR. The gate requires the incremental.exact.done marker, records rank-refresh markers and bounded log tails, and cleans only paths it creates.
This does not change defaults. The current implementation is expected to fail the default 10x threshold until exact-delta post-passes are scoped or marked stale safely.
Signed-off-by: Andrew Hundt <ATHundt@gmail.com>