RosettaOS (macOS 27+) x86_64 coverage gaps beyond ipsw extract

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#1250 (+ #1252) route ipsw extract --dyld to the Cryptex1,RosettaOS DMG for x86_64-family caches on macOS 27+. Other code paths that read DSCs or x86_64 content still only look at SystemOS/FileSystem and will silently miss data on macOS 27+:

• ipsw diff — internal/diff resolves volumes via its own typ→cryptex mapping (ipsw_artifacts.go: fs/sys/app/exc only), and dscJob walks the sys (or pre-cryptex fs) volume. macOS 27 diffs will silently omit x86_64 DSC changes. Needs a rosetta volume type plumbed through the volume session, per-side resolution in dscVolumeFor/InputHash (mixed 26↔27 pairs), and a cache-version bump.
• Remote OTA DSC extraction — ExtractFromRemoteCryptex matches cryptex-system-(arm64e?|x86_64h?); unverified whether macOS 27 OTAs ship a separate rosetta cryptex member. Check naming once a 27 OTA is available.
• internal/search / macho search / anything mounting SystemOS for x86_64 binaries — x86_64 content inside the RosettaOS cryptex won't be scanned.
• AEA-encrypted RosettaOS — beta IPSWs ship it unencrypted; confirm the .dmg.aea path works once release IPSWs ship (per @supervacuus in feat(extract): support RosettaOS x86_64 DSC #1250, untestable today).