Rebuild of the fs-overwrite-diff-bound branch on current master. Adds the
diffBasisMaxBytes Config field (10 MiB default, capped by runtime
allocation/decode limits), gates both overwrite sides, and reads the prior
basis from the bounded opened descriptor in cancellation-aware chunks; any
post-stat size change returns a null basis. Also pins the one-extra-byte
growth probe with a regression and drops the now-covered v8 ignore.
