AACS 2.0: P-256 curve, SHA-256 ECDSA, bus decryption pipeline

- Added P-256/secp256r1 curve constants
- Added AACS 2.0 LA public key for drive cert verification
- Implemented P-256 ECDSA sign/verify (SHA-256)
- Implemented P-256 ECDH for bus key derivation
- Added aacs2_authenticate() entry point (falls back to AACS 1.0)
- Extended HostCert with optional v2 credentials
- Added sha2 dependency
- Added keydb updater (HTTP GET via raw TCP, zip/gzip extraction)
- 4 new P-256 tests: generator, order, ECDSA, ECDH — all passing
- 32 total tests passing

src/aacs/mod.rs

@@ -44,8 +44,14 @@ pub struct DeviceKey {
 /// Host certificate + private key for AACS SCSI authentication.
 #[derive(Debug, Clone)]
 pub struct HostCert {
+    /// AACS 1.0: 20 bytes. AACS 2.0: 32 bytes.
     pub private_key: [u8; 20],
-    pub certificate: Vec<u8>, // 92 bytes
+    /// AACS 1.0: 92 bytes. AACS 2.0: 132 bytes.
+    pub certificate: Vec<u8>,
+    /// AACS 2.0 host private key (P-256, 32 bytes). None for AACS 1.0 only.
+    pub private_key_v2: Option<[u8; 32]>,
+    /// AACS 2.0 host certificate (type 0x11). None for AACS 1.0 only.
+    pub certificate_v2: Option<Vec<u8>>,
 }
 
 /// A per-disc entry from the key database.
@@ -201,6 +207,8 @@ impl KeyDb {
         Some(HostCert {
             private_key: parse_hex20(priv_str)?,
             certificate: parse_hex(cert_str)?,
+            private_key_v2: None,
+            certificate_v2: None,
         })
     }