feat(subjektiv): share domain and add explicit standalone Subject hosts

crates/feature-storage/Cargo.toml

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+[package]
+name = "feature-storage"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+publish = false
+
+[dependencies]
+rusqlite.workspace = true
+thiserror.workspace = true
+uuid = { workspace = true, features = ["v7"] }
+
+[dev-dependencies]
+tempfile.workspace = true

crates/feature-storage/README.md

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+# feature-storage
+
+Neutral Host-owned SQLite lifecycle manager for trusted Features. Owns placement,
+WAL/connection settings, transaction serialization, migrations, snapshots,
+restore/delete fencing, and shutdown. Features receive a managed
+`FeatureDatabase` after registration. Worker/model input never selects a database
+path or receives SQL access.
+
+Use `FeatureStorage::new(trusted_root)`, `register(FeatureRegistration)`, and
+`scope(host_issued_scope_id).open(&registration)`. `ScopedFeatureStorage` and
+`FeatureDatabase::scope_id()` are neutral storage isolation APIs: a local Host's
+persisted random scope does not create a Workspace record or imply Backend
+membership. Existing placement `<root>/<scope>/features/<feature>.sqlite` and
+snapshot `workspace=` keys remain unchanged for physical compatibility.
+
+`WorkspaceFeatureStorage`, `workspace()`, `workspace_id()`, and
+`for_server_database()` are Backend compatibility aliases/helpers. They do not
+introduce a Server dependency. The Host is responsible for state-root privacy,
+symlink/path validation, and external execution leases before constructing this
+trusted manager.
+
+`configure_connection(&rusqlite::Connection)` is reusable by other Host stores
+(such as JobStore). It sets the five-second busy timeout, foreign keys ON, WAL,
+and synchronous FULL before migrations/transactions. The Feature manager uses
+this same helper; WAL lock-contention retries stay within the busy timeout.
+
+Dependencies: `rusqlite` (existing workspace backup support), `thiserror`, `uuid`;
+tests use `tempfile`.