The workspace already has cubestore (CubeBackend trait + HashBackend
= HashMap<u32,Vec<u8>>), cubefs, and cubesys — the correct foundation
the PDF spec describes. cube-store-raw/ and cube-fuse-proxy/ were
scaffolded as redundant duplicates on the wrong architecture; they
broke `cargo check` (103 errors) and had no tests.
Also: add 7 direct HashBackend trait-level tests (PDF spec's
HashMap<u32,Vec<u8>> sketch) + clean up the _assert_backend_assoc
workaround + dead HashMap import in record_codec, both now unneeded
since HashBackend is exercised by real tests.
Verification: cargo test -p cubestore = 15/15 pass; cargo check
--workspace green (only pre-existing cubetrace warnings).
Co-Authored-By: Hermes Agent
