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/drm/mod.rs

@@ -246,11 +246,12 @@ mod tests {
         let uk_ro = vec![0u8; 256];
         let vid = [0u8; 16];
         let keydb = aacs::KeyDb::empty();
+        let providers: &[&dyn aacs::KeyProvider] = &[&keydb];
         let ctx_aacs = aacs::ResolveContext {
             unit_key_ro: &uk_ro,
             content_cert: None,
             volume_id: &vid,
-            keydb: &keydb,
+            providers,
             mkb: None,
         };
         let mut ctx = DrmContext {
@@ -272,12 +273,13 @@ mod tests {
         let uk_ro = vec![0u8; 256];
         let vid = [0xAAu8; 16];
         let keydb = aacs::KeyDb::empty();
+        let providers: &[&dyn aacs::KeyProvider] = &[&keydb];
         let mkb = mkb_with_variant();
         let ctx = aacs::ResolveContext {
             unit_key_ro: &uk_ro,
             content_cert: None,
             volume_id: &vid,
-            keydb: &keydb,
+            providers,
             mkb: Some(&mkb),
         };
         // Just confirm the symbol is callable; we don't assert on the