udf: honor ICB allocation-descriptor type (Short/Long/Extended AD)

read_icb_extents hardcoded an 8-byte Short-AD stride for every file. Large
BD-ROM .m2ts streams use 16-byte Long ADs; striding them as Short ADs reads
descriptor #0 correctly (length+lba align) but lands #1 in the middle of the
first Long AD (its zero impl_use bytes). The AD-list terminator (data_len==0
=> break) then fired on that zero and stopped after the first extent, so every
multi-extent title truncated at ~1 GiB. The same reader backs read_file, so
disc AACS-input files (/AACS/*.inf) and the m2ts mux extents were both
affected.

Read the ICB Tag flags (AD type) and stride 8/16/20 bytes for Short/Long/
Extended ADs accordingly; Extended ADs carry the lba at off+12.

aacs: extract trim_mkb and restore its guard so an MKB whose content length
the parser cannot determine (mkb_content_len == 0) is returned intact instead
of truncated to empty.

Regression tests: Long-AD read_icb_extents returns all extents; Long-AD
read_file returns full content; trim_mkb never zeroes an unrecognised MKB.

src/aacs/mod.rs

@@ -34,6 +34,7 @@ pub use keys::{
     derive_media_key_from_dk, derive_media_key_from_pk, derive_media_key_from_pk_walked,
     derive_vuk, disc_hash, disc_hash_hex, mkb_content_len, mkb_version, parse_content_cert,
     parse_unit_key_ro, read_mkb_from_drive, resolve_keys_v1, resolve_keys_v2, resolve_keys_v21,
+    trim_mkb,
 };
 pub use provider::KeyProvider;
 pub use variants::{