ld: public profile catalog API + emulation feature; clippy-clean

Expose the LibreDrive profile catalog as a curated public API on `ld`:
`ld::profiles()` returns the `Profiles` object (with `.get(drive_id)`) and
`ld::profile(filter)` finds one — the catalog of supported drives, queried
without unlocking. The unlock mechanism (firmware blobs, upload sequence, CDB
wire format) stays private; `LibreDrive` is pub(crate), reached only via
`all_unlockers()`.

The unlock-handshake wire format the bdemu test-emulator needs to impersonate a
drive (`UNLOCK_MARKER`, `is_unlock_read_buffer`) is behind a non-default
`emulation` feature; the `cdb` module compiles only under it. Rename the
`ScsiError` error variant to `Scsi` (matches css), gate the test-only
`load_bundled` and `SCSI_STATUS_CHECK_CONDITION`. Clippy-clean in both feature
configs; 86/89 tests pass.

Cargo.toml

@@ -7,6 +7,12 @@ license = "AGPL-3.0-only"
 description = "Unlock layer for the freemkv toolchain: the Unlocker contract + self-contained firmware/AACS/CSS unlocker modules. libfreemkv depends on this and dispatches via all_unlockers()."
 repository = "https://github.com/freemkv/freemkv-unlock"
 
+[features]
+# Off by default. Exposes ld's unlock-handshake wire format (`UNLOCK_MARKER`,
+# `is_unlock_read_buffer`) for the bdemu test-emulator, which impersonates an
+# ld-unlockable drive. Real clients never enable it and keep the generic API.
+emulation = []
+
 [dependencies]
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"