aacs: KeyProvider abstraction + PK walker + external-UK key source

Two coherent additions to the AACS resolver:

KeyProvider abstraction (provider.rs) — key material comes from pluggable
backends; KeyDb implements it (device/processing keys, host certs,
disc-by-hash / disc-by-vid lookup) plus orphan-DK parsing. ResolveContext
takes a provider array. Adds the SD-tree PK walker
(derive_media_key_from_pk_walked) and a `probe` module (km_verifies MK
oracle, mkb_* record parsers) used for offline key verification. Cvalue
record selection prefers 0x05, falls back to 0x07.

External-UK key source — the second, mutually-exclusive key source for the
keyserver path. ScanOptions/InputOptions gain `unit_key`; when set,
resolve_encryption_static skips keydb entirely and uses the caller-supplied
Unit Key directly (KeySource::ExternalUk). Disc::read_aacs_inputs exposes a
disc's Unit_Key_RO.inf + MKB so a caller can fetch the UK out-of-band; the
library makes no network call itself.

CHANGELOG: redact test-disc title in historical notes.

src/mux/resolve.rs

@@ -163,6 +163,9 @@ fn validate_network_addr(addr: &str) -> io::Result<()> {
 #[derive(Default)]
 pub struct InputOptions {
     pub keydb_path: Option<String>,
+    /// Caller-supplied Unit Key (keyserver path) — the second, mutually
+    /// exclusive key source. Takes precedence over `keydb_path`.
+    pub unit_key: Option<[u8; 16]>,
     pub title_index: Option<usize>,
     /// Skip decryption — return raw encrypted bytes.
     pub raw: bool,
@@ -181,11 +184,9 @@ pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::S
         }
         StreamUrl::Iso { ref path } => {
             validate_file_path(path, "iso")?;
-            let scan_opts = match &opts.keydb_path {
-                Some(p) => crate::disc::ScanOptions {
-                    keydb_path: Some(p.into()),
-                },
-                None => crate::disc::ScanOptions::default(),
+            let scan_opts = crate::disc::ScanOptions {
+                keydb_path: opts.keydb_path.as_ref().map(Into::into),
+                unit_key: opts.unit_key,
             };
             // FileSectorSource is the sole file-backed sector source.
             // It carries the platform-tuned SEQUENTIAL fadvise hint