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/lib.rs

@@ -85,6 +85,7 @@ pub(crate) mod identity;
 pub(crate) mod ifo;
 pub mod io;
 pub mod keydb;
+pub mod keysource;
 pub mod labels;
 pub(crate) mod mpls;
 pub mod mux;
@@ -174,6 +175,7 @@ pub use disc::{
     LabelPurpose, LabelQualifier, PatchOptions, PatchOutcome, Resolution, SampleRate, ScanOptions,
     Stream, SubtitleStream, SweepOptions, VideoStream, classify_damage,
 };
+pub use keysource::{DiscInputs, KeySource};
 
 // ─── Streams ────────────────────────────────────────────────────────────────
 //