aacs: parse HD DVD VTKF title keys at the spec's 36-byte stride

The HD DVD Title Key File (VTKF*.AACS) stores 64 title-key entries of 36 bytes
each — 1-byte BIFO + 3 reserved + 16-byte encrypted key + 16-byte binding MAC —
per AACS "HD DVD and DVD Pre-recorded Book" Table 3-8, confirmed byte-exact
against real discs (Freedom VTKF090, Dukes VTKF000: every 36-byte slot has
BIFO=0x80, a clean key, and a 0xFF binding MAC).

The parser used a 32-byte stride (a 12-byte pad in place of the 16-byte binding
MAC) with flag-based termination. That aligns entry #1 (key at offset 132, where
both strides agree) but drifts +4 bytes per entry after it and never terminates
(the previous entry's 0xFF MAC reads as a set present-flag), so it recovered a
correct key only for single-CPS-unit discs and garbage for CPS unit >=2. Every
multi-title HD DVD (Freedom, Harry Potter) was affected.

Fix: 36-byte stride, iterate the fixed 64 slots, take slots whose BIFO AV_FLG
(bit 7) is set, key at offset 4, slot index = CPS unit (skip empty slots rather
than terminate so a gap can't renumber later keys), and never read the trailing
16-byte TKF MAC as a key. Tests rebuilt on the real layout, including a full
64-entry file.

Also correct the VTKF-selection TODO in mod.rs: the AACS HD DVD Book gives the
selector explicitly (match the TKF's PLAYLIST_NAME field to the active
playlist), not the "validate against an encrypted unit" placeholder.

Reconciled against the new HD DVD reference (freemkv.org/docs/hddvd/); the spec
source is archived in freemkv-private/spec/.

CHANGELOG.md

@@ -11,8 +11,15 @@
   `AAC!` instead of `ANY!`) or ship numbered title-key files (`VTKF090.AACS` /
   `VTKF100.AACS` rather than `VTKF000.AACS`) are now handled: the AACS directory
   is located by its contents and every title-key file in it is picked up. Blu-ray
-  and UHD are unaffected. (Selecting the correct title-key file when a disc
-  carries several variants still needs verification against an encrypted HD DVD.)
+  and UHD are unaffected.
+- **HD DVD multi-title decryption reads the right keys.** The HD DVD title-key
+  file (`VTKF*.AACS`) stores its keys in 36-byte records — per the AACS HD DVD
+  specification, and confirmed byte-exact on real discs. freemkv had been reading
+  them at a 32-byte stride, which lands the first key correctly but drifts off
+  every key after it, so only single-title discs decrypted. Discs with more than
+  one protected title now recover every title's key instead of only the first.
+  (Choosing the correct title-key file when a disc carries several playlists
+  still needs verification against an encrypted HD DVD.)
 - **A dirty disc can no longer "rip clean" but decode with errors.** freemkv now
   asks the drive to *report* marginal reads instead of silently returning
   best-effort data as success — on smudged/scratched media a drive can hand back