* filer: POSIX advisory lock set primitive (phase 1)
Pure per-inode conflict/coalesce/range-split logic for fcntl byte-range
and flock whole-file locks, extracted from the mount's PosixLockTable
without its wait queue or inode-map concurrency. Owner identity is
(Sid, Owner) so the same FUSE owner on different mounts never aliases,
and ReleaseSession reaps a dead mount's locks. The owner filer will hold
one Set per inode under the per-path lock; no concurrency control here.
* test: tolerate transient FUSE invisibility in ConcurrentReadWrite
A concurrent truncating overwrite leaves a short-lived dentry/cache window
where the file is momentarily ENOENT to another opener. Retry the reads and
writes a few times before failing, as ConcurrentDirectoryOperations does.
