css: new freemkv-unlock-css plugin crate

The CSS (DVD-Video) bus-auth unlocker as a first-class external plugin, peer
to freemkv-unlock-ld. CssUnlocker impls libfreemkv::Unlocker — matches
DiscKind::Css, self-guards against the hardware (refuses NotApplicable without
issuing a CSS CDB if the drive reports a non-DVD profile), then runs the
libfreemkv bus-auth primitive (unlock_css_reads). libfreemkv keeps the bus-auth
+ keyless descramble; this crate owns the unlocker impl. Additive: a consuming
binary opts in with one register_unlocker(CssUnlocker::new()) line; the in-tree
built-in stays until clients are rewired.

css/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "freemkv-unlock-css"
+version = "1.2.0"
+edition = "2024"
+rust-version = "1.86"
+license = "AGPL-3.0-only"
+description = "CSS (DVD-Video) bus-auth unlocker plugin for libfreemkv"
+repository = "https://github.com/freemkv/freemkv-unlock"
+
+[dependencies]
+libfreemkv = "1.2.0"
+tracing = "0.1"