Restructure into freemkv-unlock workspace (ld member + room for more)

libfreemkv owns the Unlocker trait + registry; this repo holds the concrete
unlocker plugins. LibreDrive moves to the ld/ member (package freemkv-unlock-ld
unchanged). Workspace root + repo README describe the general-unlocker model;
adding an unlocker = a new member + one register_unlocker line in the consumer.
libfreemkv dep is the crates.io version; local dev resolves it via the
gitignored .cargo/config.toml patch.

ld/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "freemkv-unlock-ld"
+version = "1.0.0-rc.3"
+edition = "2024"
+rust-version = "1.86"
+license = "AGPL-3.0-only"
+description = "LibreDrive unlocker plugin for libfreemkv (firmware unlock for MediaTek MT1959 drives)"
+repository = "https://github.com/freemkv/freemkv-unlock"
+
+[dependencies]
+libfreemkv = "1.0.0-rc.3"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+base64 = "0.22.1"
+tracing = "0.1"