`ipsw download ipsw --kernel` now fetches firmware keys from
theapplewiki and decrypts encrypted kernelcaches inline. Unencrypted
members in the same IPSW pass through unchanged.
- pkg/img4: DecryptPayload reuses Payload.GetData for decompression,
removing the duplicate LZSS/LZFSE branches.
- pkg/kernelcache: ParseImg4Data switches to img4.ParsePayload and
exports ErrEncryptedKernelCache so callers can detect the missing-key
case via errors.Is.
- internal/commands/extract: new keyed remote path with all-or-nothing
preflight; the encryption-status peek lets unencrypted variants
succeed even when the wiki has no entry for them.
closes#1193
blacktop