fix(slayerfs): unify session usage in unprivileged mount

[Ivanbeethoven]

Motivation

• Allow filesystem code to send FUSE kernel cache invalidation notifications after mutating operations so clients see updates promptly.
• Propagate a clonable Notify handle from the FUSE Session/MountHandle into the VFS layer so async operations can trigger cache invalidation without tight coupling.

Description

• Made MountHandle carry a notify: Notify and added MountHandle::notify() to return a clonable notifier.
• Made Session::notify() public (replacing get_notify) and switched internal uses (e.g. handle_poll) to call the public method.
• In slayerfs::mount_vfs_unprivileged, create the Session first and call fs.set_fuse_notify(session.notify()).await so the VFS receives the notifier before mounting.
• Added fuse_notify: Arc<RwLock<Option<FuseNotify>>> to VFS and initialized it in the constructor, plus set_fuse_notify to store the notifier.
• Added VFS::invalidate_entry_cache(parent, name) and VFS::invalidate_inode_cache(inode) which call into the stored Notify when present.
• Inserted calls to invalidate_entry_cache or invalidate_inode_cache in FUSE operation handlers that mutate namespace or metadata (setattr, mkdir, create, link, symlink, unlink, rmdir, rename) so kernel caches are invalidated after successful changes.
• Minor async/locking import adjustments (RwLock) and small control-flow fixes to return Ok(()) where appropriate after performing invalidations.

Testing

• Built the workspace with cargo build --workspace to verify compilation; build succeeded.
• Ran unit tests for the filesystem crate with cargo test -p slayerfs; tests passed locally.
• Ran unit tests for the rfuse3 crate with cargo test -p rfuse3; tests passed locally.

link to #551

Codex Task