Introduces direct OTA file diffing without requiring pre-extraction.
Users can now pass `.ota`/`.aea` files directly to `ipsw diff` alongside
existing IPSW and directory input modes.
Key additions:
- `internal/diff/ota_inputs.go`: OTA-specific input handling including
cryptex mounting, kernelcache extraction, firmware parsing, iBoot
parsing, and feature flag collection from mounted volumes
- `inputModeOTA` enum variant in `directory_inputs.go`
- `--key-db`, `--key-val`, and `--insecure` flags on `diff` command for
AEA-encrypted OTA decryption
- AEA key database lookup mirroring the `ota` command's key resolution
- Magic-byte-aware error handling: AEA/AA files propagate decryption
errors while ambiguous ZIPs fail silently (not-an-OTA)
- Validation rejects partial/delta and RSR OTAs with actionable messages
- Comprehensive unit tests covering all new helpers
blacktop