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

Cargo.toml

@@ -12,6 +12,7 @@ categories = ["hardware-support", "multimedia"]
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 sha1 = "0.10"
+sha2 = "0.10"
 aes = "0.8"
 cbc = "0.1"
 flate2 = "1"