fix(cubefs): make FUSE mount a durable view of cube-server daemon store

The cubefs-mount --socket path was never actually built: CubeFs<B> is
generic, so the --socket (DaemonBackend) and default (HashBackend) arms
of the match were incompatible types (E0308), and --features mount failed
to compile. The running binary was therefore the in-memory build, so
--socket was silently ignored and every FUSE write went to RAM and never
reached the daemon (rawget/rawkeys returned none / 0 keys, WAL stayed 0).

Fix: type-erase the backend. Add  (forwards to inner) in cubestore, and build
 in cubefs-mount via
Box::new(DaemonBackend::new(p)) / Box::new(HashBackend::new()). Keeps
cubefs free of a cubesys dep (acyclic graph).

Verified end-to-end on host (shared code path as the VM): a FUSE write
via  lands in the daemon store (rawget returns the
record, 5 keys present, WAL grows), and survives a  of the
daemon + relaunch with the same --store (byte-identical read-back).

Also includes (from RESUME-cubefs-daemon.md): cubesys raw* command family
(rawget/rawput/rawdel/rawkeys/rawscan + parse/hex helpers) and the
DaemonBackend client + smoke tests. Report/verification docs added.

Note: VM cubefs.service still mounts in-memory (no --socket); update the
unit to  as a
follow-up so the deployed VM FUSE is durable too.

cubefs/src/lib.rs

@@ -39,6 +39,7 @@
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]
 
+pub mod backend;
 pub mod nullspace;
 pub mod path;
 pub mod vfs;
@@ -46,6 +47,7 @@ pub mod vfs;
 #[cfg(feature = "mount")]
 pub mod fuse;
 
+pub use backend::DaemonBackend;
 pub use nullspace::{Acl, JournalEntry, JournalOp, NullSpace, VolumeMeta};
 pub use path::{czyx_to_ino, ino_to_czyx, parse_path, render_path, PathError, ROOT_INO};
 pub use vfs::{Attr, CubeFs, FsError, Kind};