* fix(ec_bitrot): cap sidecar block_size in ValidateBitrotManifest
A sidecar loaded from disk (or supplied via a backfill/peer RPC) could carry a
huge power-of-two block_size that passed validation, then force a multi-GiB
scratch-buffer allocation in scrub/verify. Add a shared MaxBitrotBlockSize
(64 MiB) constant, enforce it as an upper bound in isPow2MultipleOf1MiB, and
derive the volume flag cap from the same constant so they cannot drift.
* fix(ec_bitrot): don't destroy a valid destination sidecar on an optional copy
writeToFile opened the destination with O_TRUNC before knowing whether the
source had the file, so an optional copy (ignoreSourceFileNotFound) from a source
that lacks the .ecsum truncated and then removed a valid pre-existing destination
sidecar. Stage the optional copy into a temp sibling and commit it with an atomic
rename only when the source actually delivered the file; a missing source is now
a no-op. Mandatory copies keep their in-place behavior.
Chris Lu
80dd3b2621