Add P25 voice decryption support (AES-256, DES-OFB, ADP/RC4)

[blantonl]

Summary

• Ports the complete OP25 decryption stack into trunk-recorder's bundled op25_repeater library
• Supports real-time decryption of encrypted P25 voice traffic for Phase 1 (FDMA) and Phase 2 (TDMA), provided the encryption key is known and configured
• Encryption keys are specified per-system via a dedicated JSON file referenced in config.json as "encryptionKeysFile"
• Three algorithms supported: AES-256 (algid 0x84), DES-OFB (algid 0x81), ADP/RC4 (algid 0xAA)

Key changes

• New crypto algorithm files (p25_crypt_aes, p25_crypt_des, p25_crypt_adp) with abstract base class and factory pattern
• Wired crypt_key()/crypt_reset() through p25_frame_assembler_impl to p25p1_fdma and p25p2_tdma — the critical missing link that was preventing decryption from working
• Per-system encryption key file loading from JSON config
• Keys are passed to the frame assembler on each call start

Configuration

Add encryptionKeysFile to your system config:

{
  "systems": [{
    "type": "p25",
    "encryptionKeysFile": "/path/to/keys.json",
    ...
  }]
}

Key file format (see examples/encryption_keys.json):

{
  "keys": [
    { "keyid": "0x0001", "algid": "0x84", "key": "0123456789ABCDEF..." },
    { "keyid": "0x0002", "algid": "0x81", "key": "0123456789ABCDEF" },
    { "keyid": "0x1b50", "algid": "0xaa", "key": "0123456789" }
  ]
}

Test plan

This PR requires significant testing across all three encryption algorithms before merging.

• Test AES-256 (algid 0x84) decryption with known key on a live P25 system — verify clear audio output
• Test DES-OFB (algid 0x81) decryption with known key on a live P25 system — verify clear audio output
• Test ADP/RC4 (algid 0xAA) decryption with known key on a live P25 system — verify clear audio output
• Test Phase 1 (FDMA) encrypted calls
• Test Phase 2 (TDMA) encrypted calls
• Verify unencrypted calls are unaffected when keys file is configured
• Verify behavior when no encryptionKeysFile is specified (should work as before)
• Verify behavior with incorrect/mismatched keys (should produce garbled audio, not crash)
• Build test on Linux (Ubuntu 22.04+)
• Build test on macOS

[TheGreatCodeholio]

I like this idea to add the functionality for decryption. Unsure how to best test this. I guess best way is to just make sure non-encrypted calls aren't affected and all other current functionality stays the same.

[Dygear]

I have a system I can test this on.