aacs: discover HD DVD AACS dir + title-key files instead of hardcoding /ANY!/VTKF000

The HD DVD AACS directory name and title-key filename are chosen by the
authoring house, but the resolver hardcoded a single spelling
(/ANY!/VTKF000.AACS, /ANY!/MKBROM.AACS, /ANY!/CONTENT_CERT.AACS). Real discs
diverge: Freedom (Memory-Tech) names its AACS dir AAC! and ships VTKF090.AACS
+ VTKF100.AACS; Harry Potter carries VTKF000/001/002/099. On such a disc the
hardcoded path finds nothing, so no MKB/title-key/cert is read and decryption
silently can't engage.

Replace the fixed HD DVD path constants with structural discovery:
- find_hddvd_aacs_dir() locates the AACS dir as the root child dir ending in
  '!' that contains MKBROM.AACS (so the ..._BAK mirror is skipped; the dozens
  of decoy advanced-content '!' dirs are excluded by the MKBROM.AACS guard).
- role_paths(udf, role) builds the ordered candidate list per role: the static
  BD/UHD /AACS/ paths first, then the discovered HD DVD files — MKBROM.AACS,
  CONTENT_CERT.AACS, and every VTKF*.AACS (sorted), not just VTKF000.
- read_first() is now generic over &str / String so it takes the Vec<String>.

BD/UHD unaffected (no '!' dir → discovery returns None, list is the /AACS/
constants exactly as before). Verified on real Freedom (AAC!/VTKF090+100) and
Dukes (ANY!/VTKF000) ISOs; unit tests cover both shapes.

Open item (TODO(hddvd-encrypted)): when a disc has multiple VTKF variants the
correct one must be chosen by validating its VUK-derived key against a real
encrypted unit rather than first-that-reads. Blocked on obtaining a genuinely
encrypted HD DVD image — all HD DVD ISOs on hand are already-decrypted rips.

CHANGELOG.md

@@ -4,6 +4,15 @@
 
 ### Fixed
 
+- **HD DVD AACS key files are now found on every disc, not just the common
+  layout.** The AACS directory and title-key filename on HD DVD are chosen by
+  the authoring house, and freemkv previously assumed one fixed spelling
+  (`/ANY!/VTKF000.AACS`). Discs that name their AACS directory differently (e.g.
+  `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.)
 - **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