libfreemkv 1.7.4: single drive-owned AACS bus-removal point

Relocate AACS bus decryption to one spot at the bottom of the sector chain:
the Drive's SectorSource impl applies decrypt_bus_in_content (gated to
encrypted-content ranges), with the stage decided once from the unlock/
handshake result (BusStage::from_read_data_key). Remove read_data_key from
DecryptKeys::Aacs and AacsState and the two downstream decrypt_bus call sites;
every reader (key sampler, mux, unit-key validation) now sees already-de-bussed
content and never reasons about bus encryption. Add BusRemovalSectorSource
stream + tests; keep the AacsBusKeyUnavailable fail-loud gate.

tests/pass_n_patch_fix.rs

@@ -88,7 +88,6 @@ fn decrypt_keys_is_encrypted_variants() {
 
     let aacs = DecryptKeys::Aacs {
         unit_keys: vec![],
-        read_data_key: None,
         format: libfreemkv::disc::ContentFormat::BdTs,
     };
     assert!(aacs.is_encrypted());