0.27.4: lookup-free Key API (Disc::decrypt_with) + E8005 deferred-mux fix

Add libfreemkv::Key (non_exhaustive; Unit wired) + Disc::decrypt_with(Key) — the
caller resolves a key from any source and hands it in; the library decrypts with
no keydb/network of its own. inject_unit_keys now synthesizes a minimal
ExternalUk AACS state when a scan built none (no keydb at scan time), so a disc
keyed at sweep but scanned without a keydb still decrypts from its mapfile UK
instead of reporting E8005 and deferring the mux forever. Tests cover the
synthesize, replace, empty->None, and decrypt_with paths.

src/lib.rs

@@ -170,9 +170,9 @@ pub use decrypt::{DecryptKeys, decrypt_sectors, decrypt_threads, set_decrypt_thr
 // prefix at the crate root to keep both addressable.
 pub use disc::{
     AacsState, AudioChannels, AudioStream, Clip, Codec, ColorSpace, ContentFormat, DamageSeverity,
-    Disc, DiscFormat, DiscId, DiscTitle, Extent, FrameRate, HdrFormat, KeySource, LabelPurpose,
-    LabelQualifier, PatchOptions, PatchOutcome, Resolution, SampleRate, ScanOptions, Stream,
-    SubtitleStream, SweepOptions, VideoStream, classify_damage,
+    Disc, DiscFormat, DiscId, DiscTitle, Extent, FrameRate, HdrFormat, Key, KeySource,
+    LabelPurpose, LabelQualifier, PatchOptions, PatchOutcome, Resolution, SampleRate, ScanOptions,
+    Stream, SubtitleStream, SweepOptions, VideoStream, classify_damage,
 };
 
 // ─── Streams ────────────────────────────────────────────────────────────────