AACS: add KeySource trait + DiscInputs; Media key carries a candidate pool

The library now describes the key-source abstraction it consumes, while the
concrete sources live in a companion crate. A KeySource looks a disc up from
its DiscInputs (disc hash, volume id, MKB, Unit_Key_RO.inf) and returns a Key;
the library derives down and decrypts. Sources do no derivation.

Disc::inputs() exposes those inputs from a scanned disc.

Key::Media now carries a Vec of candidate media keys rather than one: a media
key is MKB-scoped, so a source hands its whole pool and the library picks the
one that verifies against the disc's MKB. That mirrors Device/Processing and
keeps key selection (a derivation step) inside the library.

src/keysource.rs

@@ -0,0 +1,49 @@
+//! Key sources — the lookup layer that hands libfreemkv a [`Key`].
+//!
+//! libfreemkv performs NO key lookup. An application resolves a key for a disc
+//! through one or more [`KeySource`]s, each a dumb adapter over a backing store
+//! (a keydb file, a key server, the mapfile cache): given the disc's
+//! [`DiscInputs`] it returns the raw [`Key`] at whatever level it holds. The
+//! library then derives down and decrypts via `Disc::decrypt_with`.
+//!
+//! Source implementations are published in the companion `freemkv-keysources`
+//! crate — keeping all key *policy* (which store, which order, online vs local)
+//! out of the library while all key *mechanism* (the AACS derivation chain)
+//! stays in it.
+
+use crate::disc::Key;
+use crate::error::Result;
+
+/// The public AACS inputs a key source needs to look a disc up. Captured at
+/// scan; contains no secrets — only the disc identity and the on-disc AACS
+/// structures a source or key server may key on.
+#[derive(Debug, Clone)]
+pub struct DiscInputs {
+    /// SHA-1 of `Unit_Key_RO.inf`, `0x`-prefixed hex. The value a keydb keys
+    /// its per-disc entries by, and a key server identifies the disc with.
+    pub disc_hash: String,
+    /// Volume ID (16 bytes). `[0u8; 16]` when no authenticated handshake ran
+    /// (e.g. an ISO/mapfile flow), which disables VID-keyed lookups.
+    pub volume_id: [u8; 16],
+    /// Raw MKB bytes. Empty when not captured.
+    pub mkb: Vec<u8>,
+    /// Raw `Unit_Key_RO.inf` bytes. Empty when not captured.
+    pub unit_key_ro: Vec<u8>,
+}
+
+/// A key source: given a disc's [`DiscInputs`], look up a [`Key`].
+///
+/// Dumb by contract — a source queries its backing store and returns the raw
+/// key at whatever level it has (device / processing / media / volume / unit).
+/// It performs NO AACS derivation; `Disc::decrypt_with` derives down. That
+/// keeps every derivation step in one place (the library) across AACS
+/// 1.0 / 2.0 / 2.1 / 2.x.
+pub trait KeySource {
+    /// Look up a key for this disc.
+    ///
+    /// - `Ok(Some(key))` — a key was found; the caller hands it to
+    ///   `Disc::decrypt_with`.
+    /// - `Ok(None)` — this source has nothing for the disc; try the next one.
+    /// - `Err(_)` — the source itself failed (I/O, network, parse).
+    fn resolve(&self, inputs: &DiscInputs) -> Result<Option<Key>>;
+}