feat(offline-cache): scaffold qbz-offline-cache crate with the tauri-free core

Slice 0 of the offline-cache port (extraction per ADR-006). Creates the new
qbz-offline-cache workspace crate and moves the tauri-free leaf modules out of
src-tauri/src/offline_cache/ verbatim:

- types: the shared serde DTOs (was inline in offline_cache/mod.rs)
- db: the SQLite cached_tracks index (super:: -> crate::types)
- secret_vault, cmaf_store, path_validator: moved verbatim
- maintenance: super:: refs re-pointed to crate::

src-tauri is untouched and keeps its own copy, so the production build stays
green; the re-point + delete happens at the end of Slice 0. Crate compiles
in isolation and all 5 unit tests pass.

Remaining for Slice 0: playback pure-fn split, metadata/migration/purge
(re-point crate::api/crate::library -> qbz_qobuz/qbz_library), the downloader
+ CacheEvent/CacheEventSink abstraction, the OfflineCacheState struct, then
re-wire the Tauri commands and delete the old module.

cc

crates/Cargo.toml

@@ -15,6 +15,7 @@ members = [
     "qbz-cast",
     "qbz-cmaf",
     "qbz-secrets",
+    "qbz-offline-cache",
     "qconnect-core",
     "qconnect-protocol",
     "qconnect-transport-ws",