libfreemkv: clip-anchored AACS unit gate + consolidate key mechanism

The AACS unit-alignment gate measured `lba % 3` against absolute disc LBA 0,
but aligned units are anchored at each clip's encrypted-region start. A clip
whose start_lba is not 3-aligned had its readable units wrongly rejected with
"Decryption failed" (the big-title-only failure on some Blu-rays). One
canonical clip-anchored helper (`aacs::is_unit_aligned`) is now the single
source of truth for the decrypt-on-read gate; both mux read paths set the
per-extent `unit_base = start_lba` via a new `SectorSource::set_unit_base`.

Also moves key *mechanism* into the library: the encrypted sample reader
(`read_encrypted_units`) and the candidate-key resolution loop
(`resolve_and_apply`) now live here, so a key source is purely a lookup.
Regression test covers a clip based at a non-3-aligned LBA.

src/lib.rs

@@ -194,7 +194,7 @@ pub use disc::{
     KeyOrigin, LabelPurpose, LabelQualifier, PatchOptions, PatchOutcome, Resolution, SampleRate,
     ScanOptions, Stream, SubtitleStream, SweepOptions, VideoStream, classify_damage,
 };
-pub use keysource::{DiscInputs, KeySource};
+pub use keysource::{DiscInputs, KeySource, read_encrypted_units, resolve_and_apply};
 
 // ─── Streams ────────────────────────────────────────────────────────────────
 //